Emergent functional materials with respect to extreme conditions

Resume : During past years, the superconducting material has run to its eve of the large scale applications. In this talk, I will present the state of the art of 2G-HTS tape and relevant key science and technical issues worldwide. Firstly, the various methods of fabricating 2G-HTS tapes and their current status at home and abroad will be summarized, then I will introduces the key processing technologies in the preparation of long tapes, including the technical routes, substrate development as well as artificial flux pinning in the superconducting films. Finally the power applications, especially in the field of HTS magnet will be mentioned.

Resume : Many recent discoveries on novel electronic states were made on 2D materials. Especially, by making artificial bilayer systems, where new electronic states such as superconductivity and ferromagnetism have been reported. This talk will discuss quantum phase transitions and Ising superconductivity induced in 2D transition metal dichalcogenides. Using ionic gating, quantum phases such as superconductivity can be induced electrostatically on many 2D materials. In transition metal dichalcogenides, Ising-like paring states can form on the unique electronic structure shown in Fig. 1 [1,2]. Also, we will discuss how to couple two Ising superconducting states through Josephson coupling by inducing superconductivity symmetrically in a suspended bilayer. This method can access electronic states with broken local inversion symmetry while maintaining the global inversion symmetry [3]. Controlling the Josephson coupling and spin-orbit coupling is an essential step for realizing many exotic electronics states predicted for the coupled bilayer superconducting system with strong spin-orbit interactions. References [1] Lu, J. M. Zheliuk O, et al., Science 350 1353 (2015). [2] Lu, J. M. Zheliuk O, et al., Proceedings of the National Academy of Sciences 115 3551 (2018). [3] Zheliuk O, Lu, J. M., et al., Nature Nanotechnology 14 1123 (2019).

Resume : The upper critical field is a fundamental measure of the strength of superconductivity in a material. It is also a cornerstone for the realization of superconducting magnet applications. The critical field arises because of the Copper pair breaking at a limiting field, which is due to the Pauli paramagnetism of the electrons. The maximal possible magnetic field strength for this effect is commonly known as the Pauli paramagnetic limit given as 0HPauli ≈ 1.86 [T/K] ∙ Tc for a weak-coupling BCS superconductor. The violation of this limit is only rarely observed. Exceptions include some low-temperature heavy fermion and some strongly anisotropic superconductors. In this presentation, we will discuss the superconductivity at 9.75 K in the centrosymmetric, cubic -carbide-type compound Nb4Rh2C1-. We find that this material has a remarkably high upper critical field of 0Hc2 ≈ 28.5 T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of 0HPauli ≈ 18.1 T. We will discuss possible origins and consequences of these extremely large magnetic fields in this material and compare it to other -carbide superconductors.

Resume : Despite exciting developments in the field of superconductivity over a period of three decades, cuprates remain the only materials which exhibit superconductivity above the liquid nitrogen boiling temperature at ambient pressure. Here we report results of the search for a possible superconducting state in tungsten oxides WO3-x in a wide range of oxygen deficiency 0< x< 1. In some samples with composition WO2.9 the signatures of superconductivity with the transition temperature Tc = 80 K were registered by means of magnetization measurements. The variation of resistivity with temperature in this compound also displays unusual features, such as a broad maximum around 230 K and a logarithmic increase of resistivity at low temperatures. In contrast to magnetization data, superconducting transition below Tc = 80 K is not detected in resistivity measurements. This indicates that the superconductivity is localized in small regions that do not percolate. A strong increase of positive magnetoresistance (MR) was found below 80 K. This effect is very similar to that observed in underdoped cuprates, where the substantial increase of MR was attributed to superconducting fluctuations in small clusters. Therefore the temperature dependence of MR supports the presence of non-percolating superconducting clusters in WO2.9 below 80 K in agreement with magnetization data. As a further avenue of research, it is especially promising to study pressure effects on normal and superconducting properties of oxygen reduced tungsten oxides. By applying hydrostatic or uniaxial pressure, one can tune the band structure and coupling between crystallographic shear planes, where superconducting regions are presumably localized. It is expected that by improving the coupling and percolation between superconducting clusters, bulk superconductivity and zero resistance state might be achieved.

Resume : Combining magnetism and semiconductor enables the development of magnetic semiconductors, a promising way to realize spintronic applications based on the use of both charge and spin degrees of freedom. For the classic diluted magnetic semiconductor (DMS) (Ga,Mn)As, its highest Curie temperature Tc is about 200 K, still far below room temperature. One way to solve this problem is to develop the diluted magnetic semiconductors with match doping. Here, we propose a method to realize DMS with p- and n-type carriers by choosing host semiconductors with narrow band gaps [1]. By density function theory and quantum Monte Carlo simulation, we demonstrate such semiconductors using Mn-doped BaZn2As2, which has a band gap of 0.2 eV. In addition, the Tc in Mn-doped BaZn2Sb2 is predicted to be higher than Tc = 230K in Mn-doped BaZn2As2 in the recent experiment. We also predict the DMS Cr-doped BaZn2As2 with stable ferromagnetism in p- and n-type carriers [2]. Our results show that the origin of high Tc in (Ga,Fe)Sb and (In,Fe)Sb is not due to the carrier induced mechanism because Fe3+ does not introduce carriers, and predict the DMS Cr-doped InSb, InAs, and GaSb wtih possibly high Tc [3]. Another solution is to develop two-dimensional Van der Waals ferromagnetic semiconductors. By density-functional-theory calculations, we predict several stable two-dimensional (2D) Van der Waals ferromagnetic semiconductors with high Tc, including strain-induced 2D room-temperature ferromagnetic semiconductor Cr2Ge2Se6 [4], 2D room-temperature ferromagnetic semiconductors PtBr3 and PdBr3, that could exhibit a high-temperature quantum anomalous Hall effect [5], 2D ferromagnetic semiconductors TcSiTe3, TcGeSe3, and TcGeTe3 with high Tc around 200–500 K [6], 2D Janus semiconductor Fe2Cl3I3 with both spin and charge polarizations and the highly sensitive strain-controlled magnetic states [7], 2D two-dimensional magnetic semiconductor heterostructure Cr2Ge2Te6/PtSe2 with enhanced Tc above room temperature [8], 2D Kagome magnetic semiconductors Co3Pb3S2, Co3Pb3Se2, Co3Sn3Se2 with quantum anomalous Hall effect with high Chern number and large band gap [9], and 2D magnetic semiconductors MnBi2Te4, MnBi2Se2Te2, MnBi2S2Te2 with electric field induced topological phase transition and large enhancements of spin-orbit coupling and Curie temperature [10]. Reference: [1] B. Gu and S. Maekawa, Phys. Rev. B 94, 155202 (2016). [2] B. Gu, and S. Maekawa, AIP Advances 7, 055805 (2017). [3] J. Y. You, B. Gu, S. Maekawa, and G. Su, Phys. Rev. B 102, 094432 (2020). [4] X. J. Dong, J. Y. You, B. Gu, and G. Su, Phys. Rev. Applied 12, 014020 (2019). [5] J. Y. You, Z. Zhang, B. Gu, and G. Su, Phys. Rev. Applied 12, 024063 (2019). [6] J. Y. You, Z. Zhang, X. J. Dong, B. Gu, and G. Su, Phys. Rev. Research 2, 013002 (2020). [7] Z. Zhang, J. Y. You, B. Gu, and G. Su, J. Phys. Chem. C 124, 19219 (2020). [8] X. J. Dong, J. Y. You, Z. Zhang, B. Gu, and G. Su, Phys. Rev. B 102, 144443 (2020). [9] Z. Zhang, J. Y. You, X. Y. Ma, B. Gu, and G. Su, Phys. Rev. B 103, 014410 (2021). [10] J. Y. You, X. J. Dong, B. Gu, and G. Su, Phys. Rev. B 103, 104403 (2021).

Resume : The vast majority of complex solids are prepared at high temperatures under reaction conditions that apply ‘thermodynamic control’ to the selection of reaction products. By utilizing the ‘extreme chemical conditions’ of low-temperature, and very low pO2 it is possible to prepare a range of novel solids which are metastable and cannot be synthesised under more conventional conditions. By using these synthetic approaches it is possible prepare extended solids containing transition metal cations with unusual combinations of oxidation state and coordination geometry (e.g. square-planar Ni1+, Fe2+, Ru2+, Ir2+). In addition, these conditions can also facilitate the substitution of oxide anions by hydride anions, to form transition-metal oxyhydride phases. The differing charges of O2- and H- mean that any hydride-for-oxide anion exchange is necessarily reductive. Furthermore, the greater polarizability in combination with the lower electronegativity of hydride anions means that M-H bonds have a greater degree of covalency and orbital mixing than the corresponding M-O bonds, and as a result the band structures of oxide-hydride phases are qualitatively different to all-oxide systems, and the strength of magnetic exchange couplings is also enhanced. A further, more subtle, difference between oxide and hydride is the lack of π-symmetry valence orbitals in the later anion. This means that hydride ions cannot be involved in bond formation with metal orbitals of π-symmetry. This can have a dramatic influence on the orbital connectivity of oxide-hydride phases, particularly when there is extensive anion order, as can be seen in the highly 2-dimensional character of the transport in the high-pressure metallic state of SrVO2H. The structures and physical properties of a range of novel 3d and 3d/4d reduced transition-metal oxides and oxyhydrides will be described.

Resume : When computing phase diagram from first principles, one might need to simulate prohibitively large supercells. These supercells can be modelled using atomistic models of interactions. However, these models do not allow an explicit treatment of electronic subsystem which might be important in metallic systems at high temperature. The solution to this is a hybrid method, where the free energy of the ionic subsystem is computed by thermodynamic integration for the interatomic potential and then corrected for the electronic entropy term of the electronic subsystem. We apply this method to iron phase diagram at high pressure and temperature and demonstrate good agreement with pure ab initio method for the equilibria that do not require large number of atoms and with the experiment where large number of atoms is critical. The computed phase diagram is of large practical value.

Resume : 　 The simple ABO3 and A-site-ordered AA’3B4O12 perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an Asite- ordered perovskite PbHg3Ti4O12, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg2+ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg3Ti4O12. Thus PbHg3Ti4O12 appears to be a combinatory link from simple ABO3 perovskites to A-site-ordered AA’3Ti4O12 perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA’3B4O12-type A-siteordered perovskites, and opens a direction to search for ferroelectric materials.

Resume : In the last decade, the fusion research community presented high interest in studying candidate materials and deposition techniques applicable for the fabrication of permeation barriers which are intended to minimize the nuclear fuel permeation/inventory and corrosion. The R&D motivation arises from the necessity of the future fusion reactors as Demonstration Power Station (DEMO) to integrate an inner first wall constructed from low activation martensitic steel (e.g. EUROFER) that presents the drawback of being highly transparent to hydrogen isotopes. Moreover, the cooling system and other axes parts present high contamination risks and this imposes the use of permeation barriers to avoid a radioactive hazard. In this R&D context, we studied several oxides and metals known in the literature for their suitable permeation barrier properties. Therefore, single and co-deposited layers consisting of bulk metals (W, Be) and oxides (Al2O3, Cr2O3) were deposited utilizing RF Magnetron Sputtering @ Ar pressure and Thermionic Vacuum Arc (TVA) methods. Following the deposition process, we conducted a microstructural and microchemical evaluation, taking into consideration that different deposition conditions could influence the microstructure, morphology, and layer surfaces. Here we demonstrated that metallic, oxides, and metallic-oxides films can be deposited with a wide range of morphologies utilizing RF sputtering and TVA, as revealed by SEM measurements. Film morphologies were observed from compact (Cr2O3-Be, Al2O3, Al2O3-Be, Al2O3-W) to the homogenous distribution of particles in a dense matrix (W, Be-W, Cr2O3-W) and flakes (Be, Cr2O3). The chemical compositions were analyzed qualitatively using an energy-dispersive X-ray microanalysis (EDX) system. The EDX measurements indicate that the sputtered oxides (Al2O3, Cr2O3) contain O (at. %) at close values to the atomic ratio in stoichiometric configuration, confirming the coexistence of metal-oxide in a bulk state. The mechanical properties were evaluated by hardness, elastic modulus, and adhesion strength measurements using nano indenter and nano / micro scratch tester. Lower elastic modulus on metal-oxide co-depositions was observed while indentation hardness increased for Be and decreased for W matrix configurations. A significantly better adhesion behavior was observed for pure configurations of oxides (micro scratch) and metals (nano scratch), while co-depositions were highly sensitive to premature delamination. Additionally, we address investigations regarding the desorption of native trapped hydrogen utilizing thermal desorption spectrometry (TDS) instrument and also the bounding state of atoms at the surface of the metal-oxides configurations using X-ray photoelectron spectroscopy (XPS). Overall, a mixture of oxidized and metallic states of the constituent elements was observed in the near-surface layer. However, the Al0 unoxidized feature vanishes only for Al2O3-W configuration. The determined results will assist further optimizations of the deposition techniques to achieve dense and defect-free permeation barrier structures measurable to future laborious permeation investigations. Acknowledgment: This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS / CCCDI ? UEFISCDI, with the project number PD 192 ? 2020, within PNCDI III;

Resume : The antiperovskite structure exhibits a diverse ensemble of unconventional physical and chemical properties generated by the wide structural features inherited from their perovskites counterparts and the allowed cations at X sites. As such, the Mn3AN family [1] has shown remarkable properties that go from piezomagnetic effect to frustrated magnetism making this field of broad interest within the scientific community. Among this type of materials, the Vanadium-based antiperovskite family, V3AN, has been also reported with A = Ga, Ge, As, and P in which, for A = P and As, superconducting properties were found [2]. Nevertheless, the V3AN previously studied has been reported to crystallize in a layered orthorhombic anti-postperovskite structure with space group Cmcm [3] far from the cubic symmetry shown by its Mn-based counterparts. Moreover, previous research has been mostly focused on experimental findings, such as sample synthesis and characterization of the superconducting mechanism exhibited in the anti-postperovskite phase. Then, similar cubic antiperovskite compounds among this family, where topological properties associated with intertwined magnetic structure and their symmetry, are still in pursuit. As such, in this study, we theoretically explored the structural stability and the chiral magnetism of a predicted Vanadium-based antiperovskite compound stable in the Pm3m cubic symmetry. Therefore, first-principles calculations in the framework of Density-functional theory, DFT, were carried out and were focused on the study of the structural, magnetic, and vibrational properties of a newly proposed V3AN compound. We demonstrate that the ground state structure belongs to a cubic phase and showed the structural stability of the material in this phase in terms of the phonon spectra. Furthermore, we found that the magnetic structure of the ground state symmetry shows a chiral magnetic ordering within a non-collinear antiferromagnetic state. The latter explained in terms of the magnetic frustration of the Vanadium atoms organized in a Kagomé lattice in the (111) planes. Finally, this symmetry and magnetic ordering make of this compound an ideal candidate for exploring enhanced topological properties such as Anomalous Hall conductivity. [1] Zemen, J., Mendive-Tapia, E., Gercsi, Z., Banerjee, R., Staunton, J. B., & Sandeman, K. G. (2017). Frustrated magnetism and caloric effects in Mn-based antiperovskite nitrides: Ab initio theory. Physical Review B, 95(18), 184438. [2] Wang, B., & Ohgushi, K. (2013). Superconductivity in anti-post-perovskite vanadium compounds. Scientific reports, 3(1), 1-5. [3] Wang, B. S., Cheng, J. G., Matsubayashi, K., Uwatoko, Y., and Ohgushi, K. (2014). High-pressure effects in anti-post-perovskite superconductors V3PnN x (Pn= P, As), Physical Review B, 89(14), 144510.

Resume : Wearable epidermal sensor systems have gained tremendous attention for real-time healthcare monitoring devices with different sensor geometry and device architectures. In this work, a stretchable-gradient structure is introduced to the nature-inspired kirigami-serpentine based temperature sensor with an integration of flexible printed circuit board (FPCB) and stretchable interconnection to obtain real-time monitoring of body temperature with high accuracy and sustainability under complex mechanical conditions. The electrical and mechanical properties are measured with multiple testing processes and the results show a linear response under different temperature ranges and high robustness properties over multiple strain tests. A three-layer wearable patch is designed to establish a conformable bonding to human skin and obtain wireless real-time monitoring of body temperature, thus, establishing a new approach towards real-time healthcare monitoring devices with high accuracy and stability over complex environments in the field of wearable electronics.

Resume : Two-dimensional (2D) materials have attracted a lot of interest. Their unique properties are expected to lead to new technologies. In an effort of systematic 2D materials discovery, we have been using both the top-down and the bottom-up approaches to generate 2D structures. On one hand, monolayer structures are theoretically exfoliated from layered three-dimensional structures by a topology-based algorithm. On the other hand, new 2D materials are systematically generated by chemical substitution of elements in the top-down 2D compounds by similar elements. High throughput first-principles calculations are carried out to study their physical properties. The 2D materials database, 2DMatPedia1, is publicly available and provides a good starting point for further material screening, data mining, data analysis and artificial intelligence applications2. In this talk, I will provide an update on the development and the current status of the 2D materials genome, and discuss some examples of materials discovery based on the database. By screening through 2D materials in 2DmatPedia, intrinsic ferromagnetism was discovered in several monolayer electrides or electrenes such as lanthanum bromide, in which excess electrons act as anions. The origin of magnetism and magnetic coupling in such electrenes are totally different from that in conventional magnetic materials. The electrenes were found to process relatively high Curie temperature and coercivity, suggesting potential applications in spintronic devices. In another example, a 2D tungsten nitride, which can be exfoliated from the bulk material with a low exfoliation energy, was found to be a topological metal with exotic topological states at different energy levels. Moreover, monolayer W2N3 is found to be a superconductor with the superconducting transition temperature of ∼ 22 K and a superconducting gap of about 5 meV. References: 1. http://www.2dmatpedia.org/ 2. Jun Zhou et al., 2DMatPedia, an open computational database of two-dimensional materials from top-down and bottom-up approaches, Scientific Data 6, 86 (2019) 3. Jun Zhou et al., Atomic-orbital-free intrinsic ferromagnetism in electrenes, Phys. Rev. B 102, 180407(R) (2020). 4. Jingyang You et al., Two-dimensional topological superconductivity candidate in a van der Waals layered material, Phys. Rev. B 103, 104503 (2021).

Resume : Strongly correlated materials are characterized by partially occupied d and/or f-type bands with highly localized unpaired electrons. Under compression, the hybridization of these bands causes competing valence states, and resulted in so-called mixed valency. The prototypical example is EuO. Under compression, EuO exhibits several structural transitions. It has been suggested the transformations are associated in the change in the Eu oxidation state from +2 to +3 [1.2]. We show that the isostructural transitions can be reproduced by the standard KS-DFT approach with a self-interaction correction included in the global-hybrid functional with all-electron basis sets [3]. The pressure-dependent change in the x-ray absorption spectra [2] is related to changes in the electron density of the unoccupied d-band in the different crystal fields. There is no need to invoke mixed valency to account for the experimental observations. Another example on the importance of using explicit all-electron basis sets is illustrated in the electronic structure calculations in pressure-induced changes in the 57Fe Mossbauer chemical shift of pure Fe and Fe oxides [4]. It is found that good numerical agreements with experimental measurements can be achieved with the B1WC hybrid functional. Analysis of the electronic structure shows a substantial donation of electrons from oxygen to iron at high pressure. The assignment of formal oxidation to the Fe atom becomes ambiguous under this condition. References 1. A. Jayaraman, Phys. Rev. Lett. 29, 1674 (1972) 2. N. M. Souza-Neto, et.al., Phys. Rev. Lett. 109, 026403 (2012). 3. J.K. Desmarais, et.al., Phys. Rev. Lett., 126, 196404 (2021) 4. J.K. Desmarais. et.al., J. Chem. Phys. 154, 214104 (2021).

Resume : Here, we report a series of new materials A3BX5 with quasi one-dimensional (1D) chains synthesized at the conditions of high pressure and high temperature, where A denotes Ba or La, B is 3d transition metal and X represents chalcogen or pnicogen element [1-6]. These compounds consist of face-sharing BX6 octahedral chains running in c-direction and these chains are separated with a distance >9 Å, showing a strong 1D structural character. In these quasi 1D system, it is demonstrated that when the anion of X changes from low periodic element to high periodic one their resistivity will decrease and the long range order temperature will be enhanced because of the increase of interchain electron hopping. References: [1]. Zhang, J., Wang, X.C., Jin, C.Q., et al. "High-pressure synthesis, crystal structure, and properties of iron-based spin-chain compound Ba9Fe3Se15". Phys. Rev. Mater. 5, 054606 (2021). [2]. Zhang, J., Wang, X.C., Jin, C.Q., et al. "A New Quasi One-Dimensional Compound Ba3TiTe5 and Superconductivity Induced by Pressure". NPG Asia Materials 11, 60 (2019). [3]. Zhang, J., Wang, X.C., Jin, C.Q., et al. "The synthesis of a quasi-one-dimensional iron-based Telluride with antiferromagnetic chains and a spin glass state". Inorg. Chem. 59, 5377 (2020). [4]. Zhang, J., Wang, X.C., Jin, C.Q., et al. "Ba9V3Se15: a novel compound with spin chains". J. Phys.: Condens. Matter 30, 214001 (2018). [5]. Duan, L., Wang, X.C., Jin, C.Q., et al. "High-pressure synthesis, crystal structure and physical properties of a new Cr-based arsenide La3CrAs5". Science China Materials 63, 1750 (2020). [6]. Duan, L., Wang, X.C., Jin, C.Q.,et al. "High-pressure synthesis, structure and properties of new ternary pnictides La3TiX5 (X = P, As)". Journal of Alloys and Compounds 831, 154697 (2020). [7]. Zhang, J., Wang, X.C., Jin, C.Q.,et al. "Synthesis, crystal structures, and electronic properties of one dimensional Ba9Sn3(Te1-xSex)15 (x = 0-1)", Inorganic Chemistry Frontiers 4, 1337 (2017)

Resume : Hybrid organic-inorganic compounds are an emerging family of materials that integrates organic and inorganic building blocks in their chemical structures.[1] In the recent years, these materials have exhibited several functional properties of great interest for energy applications, such as remarkable photovoltaic and optoelectronic properties for low-cost solar cells, in the case of lead halide-perovskites,[2] or enormous gas absorption capacities for hydrogen storage or CO2 capture, in the case of metal-organic frameworks (MOFs).[3] Even more recently, our research group have pioneered studies on hybrid organic-inorganic as new materials for solid-state cooling, heating and multiple energy storage.[4] Therefore, these solid compounds arise as a new alternative to the current refrigerant gases, which are mainly toxic, flammable and/or greenhouse gases. In this work, we present our more recent advances in the design and study of new hybrid materials (hybrid perovskites and ionic plastic crystals) that exhibit multiple phase transitions, which are sensible towards multiple external stimuli (such as pressure, temperature, and/or electric field).[4] These phase transitions are associated with very large thermal changes, which can occur simultaneously with large changes in their dielectric capacity. In turn, these remarkable functional properties are very promising, not only for solid-state cooling/heating, but also for multiple energy (thermal and electric) storage. Additionally, we analyse the relationships between the aforementioned properties and the structure features of these materials (such as configurational disordering, distortions, chemical modulation, among others), which will help to design future compounds with enhanced multifunctional properties of interest for eco-friendly refrigeration and heating applications. In summary, we introduce a new family of multifunctional hybrid organic-inorganic compounds whose large sensibility towards different external stimuli (pressure, temperature and/or electric fields) makes them as promising candidates for solid-state and gas-free cooling/heating technologies. References: [1] (a) O. M. Yaghi et al., Nature, 2003, 423, 705-714; (b) A. K. Cheetham and C.N.R. Rao, Science, 2007, 318, 58-59. [2] (a) M.M. Lee et al., Science, 2021, 338, 643-647; (b) J. Burschka et al., Nature, 2013, 499, 316-319. [3] (a) M. Ding et al., Chem. Soc. Rev. 2019, 48, 2783-2828; (b) M. T. Kapelewski et al., Chem. Mater., 2018, 30, 8179-8189. [4] (a) J. M. Bermúdez-García et al., Nat. Commun., 2017, 8, 15715; (b) J. M. Bermúdez-García et al., J. Phys. Chem. Lett., 2017, 8, 4419; (c) J. M. Bermúdez-García et al., J. Mater. Chem. C, 2018, 6, 9867; (d) J. Salgado-Beceiro et al., Mater. Adv., 2020, 1, 3167; (e) J. Salgado-Beceiro et al., J. Mater. Chem. C, 2020, 8, 13686-13694.

Resume : Terahertz functional components such as polarizers, modulators and filters, are crucial devices for different terahertz applications. The functional materials are the basic of functional devices. The terahertz functional materials based on the metasurfaces are paid more and more attentions in recent year due to the lack of national materials in the terahertz frequency regions. The terahertz functional materials based on the structures of metallic array are firstly studied in this work. The planar metallic structures consist of periodic array with the different unit cell of structures. The optimized design of structure parameters of metallic arrays is simulated and discussed. The spectral characteristic of metallic array structures is investigated by the terahertz time-domain spectroscopy. The terahertz modulation functionality of these metallic structures is presented and discussed. In detail, the one-dimensional metallic wire grating as a potential terahertz polarizer is designed and fabricated. The terahertz time-domain spectroscopy is used to measure the transmission of metallic grating. Moreover, two-dimensional metallic slit array as a potential terahertz attenuator is designed and analyzed. The polarization dependence of metallic slit array is discussed. The metallic cross array as a potential terahertz filter is analyzed. Finally, the terahertz vortex phase plates, which can generate the terahertz vortex beams, are designed and simulated numerically. The terahertz metasurfaces with the order structures of different functional elements are demonstrated that can provide the various functionalities for the modulation of terahertz wave. The research on the terahertz metasurfaces is helpful for the development of terahertz functional devices.

Resume : Electric pulse applied to materials changes states of matter and induces novel states. These changes may come from modification of microscopic interactions, crystalline structural, carrier density, and so on. Such photo-induced phenomena are one of current topics in strongly correlated electron systems. Mott insulating states are a good play ground for the phenomena. We firstly focus on a two-dimensional Mott insulator with square lattice, which is a parent compound of cuprate high temperature superconductors. Calculating momentum-dependent spin dynamics induced by pumping electric pulse numerically, we find an antiphase oscillation of spectral weight in the spin structure factor between momentum q=(,0) and (0,), which can be detectable by time-resolved resonant inelastic x-ray scattering [1]. The oscillation is partly originated from antiferromagnetic exchange interaction modified by the pumping pulse. We next examine photo-doping effect on a one-dimensional Mott insulator by calculating time-resolved optical conductivity numerically [2]. We find the appearance of photocarriers in the optical conductivity strongly depends on the frequency of the pumping pulse. [1] K. Tsutsui, K. Shinjo, and T. Tohyama, Phys. Rev. Lett. 126, 127404 (2021). [2] K. Shinjo, S. Sota, and T. Tohyama, in preparation.

Resume : With the intensifying development of electronics and energy storage systems (e.g. batteries for electrical vehicles) where a lot of heat is generated in operation, new high-performance heat dissipative materials need to be developed. Their goal is to help the heat dissipation in order to avoid overheating, hotspots or thermal runaways that could damage integrated circuits or threaten safety. In this study, nanocomposites made of polycarbonate (PC) and boron nitride nanotubes (BNNTs) as nanofillers are investigated (PC-BNNT systems). Boron nitride nanotubes are one of the most promising inorganic nanomaterials used to improve thermal conductivity while preserving the electrical insulation of the nanocomposites. This study focuses on incorporation of raw boron nitride nanotubes inside a polymer matrix made of polycarbonate via solution mixing. The thermal conductivity of this PC-BNNT system reaches 0.83 W/mK at 30wt% of raw BNNTs (which is approximatively 280% higher than neat PC). In addition to the thermal measurements, dispersibility of the nanotubes and their chemical affinity with the polycarbonate matrix are investigated through a SEM-EDX analysis.

Resume : Cuprate high-temperature superconductors (HTSs) have complex phase diagrams with multiple competing ordered phases. Understanding to which degree charge, spin, and superconducting orders compete or coexist is paramount for elucidating the microscopic pairing mechanism in the cuprate HTSs. In this talk, I will report some novel results of muonspin rotation (μSR) and AC susceptibility experiments on uniaxial stress effect on the static spin-stripe order and superconductivity in the La214 cuprates [1]. We find that in the cuprate system La2-xBaxCuO4 with x = 0.115, an extremely low uniaxial stress of 0.05 GPa induces a substantial decrease in the magnetic volume fraction and a dramatic rise in the onset of 3D superconductivity, from 10 to 32 K; however, the onset of at-least-2D superconductivity is much less sensitive to stress [1]. These results show not only that large-volume-fraction spin-stripe order is anti-correlated with 3D superconducting (SC) coherence, but also that these states are energetically very finely balanced. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. These results strongly suggest a similar pairing mechanism for spin-stripe order, the spatially-modulated 2D and uniform 3D SC orders, imposing an important constraint on theoretical models. [1] Z. Guguchia et. al., Phys. Rev. Lett. 125, 097005 (2020).

Resume : The rarest of the trivalent iron oxide polymorphs, i.e. ferrimagnetic ε-Fe2O3, could open new horizons for high-density magnetic storage, owing to its giant coercive field (⁓20 kOe at RT). It is moreover non-toxic, biocompatible and composed of non-critical elements, but the difficulty lies in its limited stability and challenging synthesis, in particular beyond the trace amounts enabled through conventional synthesis techniques. We have successfully developed a facile gas-phase synthesis route for amazingly stable ε-Fe2O3 thin films based on atomic layer deposition (ALD),[1] the fastest growing thin-film technology in microelectronic industry. An emerging trend in the field is to combine with ALD additional molecular layer deposition (MLD) cycles for organic layers. We have utilized the combined ALD/MLD approach to introduce with digital accuracy monomolecular benzene or azobenzene layers[2] within the ε-Fe2O3 matrix. These organic layers significantly improve the mechanical properties of the ε-Fe2O3 films without compromising their unique magnetic properties.[3] Most excitingly, the azobenzene moieties are photoresponsive, undergoing upon UV irradiation a reversible trans-cis isomerization accompanied with a considerable dimensional change, i.e. a kind of chemical pressure effect on the ε-Fe2O3 layers. Very recently we demonstrated for the azobenzene embedded ε-Fe2O3 films reversible changes in both coercivity and magnetization values with alternating irradiations of the films with UV and visible light.[4] We believe that these results are a step ahead towards the dream of switchable on-and-off type flexible hard magnets operating at room temperature. 1. Tanskanen, O. Mustonen & M. Karppinen, Simple ALD process for ε-Fe2O3 thin films, APL Mater. 5, 056104 (2017). 2. A. Khayyami, A. Philip & M. Karppinen, Atomic/molecular layer deposited iron-azobenzene framework thin films for stimuli-induced gas molecule capture/release, Angew. Chem. 58, 13400 (2019). 3. A. Philip, J.-P. Niemelä, G.C. Tewari, B. Putz, T.E.J. Edwards, M. Itoh, I. Utke & M. Karppinen, Flexible ε-Fe2O3-terephthalate thin-film magnets through ALD/MLD, ACS Appl. Mater. Interfaces 12, 21912 (2020). 4. A. Philip, Y. Zhou, G.C. Tewari, S. van Dijken & M. Karppinen, Optically controlled large-coercivity room-temperature thin-film magnets, in preparation (2021).

Resume : Magnetite is not only the oldest known magnetic material, but also the classical (1939) example of a metal-insulator transition, at TVerwey=120 K [1]. This transition has been attributed to charge (iron valence) ordering, which might be ferroelectric as proposed in several reports based on electric characterization [2]. Dielectric spectroscopy indicates relaxor ferroelectric characteristics, with polarization switching observable only at low T and in a suitably chosen time-window [3]. The very complex low T crystal structure had been only solved recently, and found to be polar [4]. Intrinsic ferroelectricity additionally requires that this polar structure can be switched by an applied electric field. Given the relaxor characteristics, structural switching can only be observable in the proper time-window. I will present a time-resolved x-ray diffraction study on single crystalline magnetite with electric field pulses applied in-situ that indicates that structural switching does occur for sufficiently large electric fields [5]. [1] E.J.W. Verwey, Nature 144, 327 (1939). [2] e.g. G.T. Rado et al., Phys. Rev. B 12, 5166 (1975); K.Kato et al., J. Magn. Magn. Mater. 31-34, 783 (1983); Y. Miyamoto et al., Ferroelectrics 93, 301 (1989); M. Alexe et al., Adv. Mater. 21, 4452 (2009). [3] F. Schrettle, S. Krohns, P. Lunkenheimer, V.A.M. Brabers, and A. Loidl, Phys. Rev. B 83, 195109 (2011). [4] M.S. Senn, J.P. Wright, and J.P. Attfield, Nature 481, 173 (2012). [5] M. Angst, S. Adiga, S.Gorfman, M. Ziolkowski, J. Strempfer, C. Grams, M. Pietsch, and J. Hemberger, Crystals 9, 546 (2019).

Resume : The anomalous Hall effect (AHE) in conventional ferromagnetic metals is proportional to the net magnetization and was expected to vanish in antiferromagnets. However, it is now well understood that the existence of the AHE is constrained by symmetry and is not necessarily incompatible with AFM order. This principle has been exemplified by the large AHE predicted and realized in various noncollinear AFMs, such as hexagonal Mn3Sn/Ge and cubic Mn3Ir/Pt, etc. where the Mn atoms form a kagome lattice along different crystalline planes. In these materials, geometric frustration and local symmetries of the magnetic atoms to gather lead to a chiral noncollinear magnetic order with the essential symmetry breaking for finite AHE, which is also reflected by the small net moment in certain crystalline directions. The large AHE in the hexagonal Mn3Sn/Ge was argued to be due to Weyl points near the Fermi energy. Technologically, the robust AHE at room temperature in these systems makes them attractive for potential applications in spintronics solely based on AFM. Meanwhile, it is interesting to search for similar anomalous Hall antiferromagnets (AHE AFMs) beyond binary Mn compounds, which may have other superior properties. Anti-perovskite manganese nitrides Mn3AN, have received a lot of attention due to a number of unusual physical properties, such as giant negative thermal expansion, temperature independent resistivity, and barocaloric effects. All of these phenomena are closely related to the first order phase transition from a high temperature paramagnetic cubic phase into a low temperature noncollinear AFM phase with the so-called Γ4g and Γ5g magnetic structures. It is interesting to notice that the Mn atoms form a 1/4-depleted fcc lattice in these manganese antiperovskites, whose (111) planes are kagome lattices. Therefore one would expect there to be AHE if the AFM order of the Mn moments is similar to these binary compounds. We report the topological AHE in antiperovskite Mn3NiN with substantial doping of Cu on the Ni site (i.e. Mn3Ni1−xCuxN), which stabilizes a noncollinear antiferromagnetic (AFM) order compatible with the AHE. Observed on both sintered polycrystalline pieces and single crystalline films, the AHE does not scale with the net magnetization, contrary to the conventional ferromagnetic case. The existence of the AHE is explained through symmetry analysis based on the Γ4g AFM order in Cu doped Mn3NiN. DFT calculations of the intrinsic contribution to the AHE reveal the non-vanishing Berry curvature in momentum space due to the noncollinear magnetic order. The binary Mn compounds encounter potential non-wetting and interfacial oxidation problems during the epitaxial thin-film growth on oxide substrates. Thus, it is much easier to obtain high quality epitaxial Mn3Ni1−xCuxN film on oxide substrates, giving much advantage for the preparation of multilayer devices in AFM spintronics. Reference: K. Zhao et al., PRB 100, 045109 (2019)