Chiral Dirac fermion in a collinear antiferromagnet

Zhang, A.*; Deng, K.*; Sheng, J.*; Liu, P.*; Kumar, S.*; Shimada, Kenya*; Jiang, Z.*; Liu, Z.*; Shen, D.*; Li, J.*; et al.

Chinese Physics Letters, 40(12), p.126101_1 - 126101_8, 2023/12

https://doi.org/10.1088/0256-307X/40/12/126101

Pressure-modulated magnetism and negative thermal expansion in the HoFe intermetallic compound

Cao, Y.*; Zhou, H.*; Khmelevskyi, S.*; Lin, K.*; Avdeev, M.*; Wang, C.-W.*; Wang, B.*; Hu, F.*; Kato, Kenichi*; Hattori, Takanori; et al.

Chemistry of Materials, 35(8), p.3249 - 3255, 2023/04

https://doi.org/10.1021/acs.chemmater.3c00158

Hydrostatic and chemical pressure are efficient stimuli to alter the crystal structure and are commonly used for tuning electronic and magnetic properties in materials science. However, chemical pressure is difficult to quantify and a clear correspondence between these two types of pressure is still lacking. Here, we study intermetallic candidates for a permanent magnet with a negative thermal expansion (NTE). Based on in situ synchrotron X-ray diffraction, negative chemical pressure is revealed in HoFe on Al doping and quantitatively evaluated by using temperature and pressure dependence of unit cell volume. A combination of magnetization and neutron diffraction measurements also allowed one to compare the effect of chemical pressure on magnetic ordering with that of hydrostatic pressure. Intriguingly, pressure can be used to control suppression and enhancement of NTE. Electronic structure calculations indicate that pressure affected the top of the majority band with respect to the Fermi level, which has implications for the magnetic stability, which in turn plays a critical role in modulating magnetism and NTE. This work presents a good example of understanding the effect of pressure and utilizing it to control properties of functional materials.

Orientation of silicon phthalocyanine thin films revealed using polarized X-ray absorption spectroscopy

Sekiguchi, Tetsuhiro; Baba, Yuji; Shimoyama, Iwao; Hirao, Norie; Honda, Mitsunori; Deng, J.*

Photon Factory Activity Report 2012, Part B, P. 68, 2013/00

The orientation of organic semiconducting molecules on surfaces plays a crucial role in improving the properties of electronic devices. We prepared thin films of silicon phthalocyanine dichroride (SiPcCl) on graphite spin-cast followed by heating to 350C at ambient condition. We investigate the molecular orientation of the films using angle-dependent near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy. Si 1s NEXAFS spectra for the thin films after annealing are very different each other, indicating that the corresponding reactions thus the structures of final products are different. The structures of the reaction products are estimated with the help of ab initio molecular orbital calculations that fit the observed NEXAFS spectra.

Orientation of Si phthalocyanine investigated by X-ray absorption spectroscopy and molecular orbital calculation

Sekiguchi, Tetsuhiro; Baba, Yuji; Shimoyama, Iwao; Honda, Mitsunori; Hirao, Norie; Narita, Ayumi; Deng, J.*

Surface and Interface Analysis, 42(6-7), p.863 - 868, 2010/06

https://doi.org/10.1002/sia.3543

Metal phthalocyanines (Pc) have attracted growing attention due to potential application as organic semiconductors or light emitters. Recently it has been pointed out that orientation of Pc molecules is one of key properties that improve the career mobility. We studied orientation property of Si-phthalocyanine thin films using synchrotron radiation. We report on following topics: (1) Orientation analysis using polarized X-rays, (2) Effect of metal substrates and annealing on orientation, (3) Analysis aided by molecular orbital method, and (4) New method to observe chemical-bond directions in nanometer scale using combined techniques of polarized X-ray absorption fine structure (XAFS) spectroscopy and photoelectron emission microscopy (PEEM).

Chemical-state-selective observations on Si-SiO at nanometer scale by photoelectron emission microscopy combined with synchrotron radiation

Baba, Yuji; Sekiguchi, Tetsuhiro; Shimoyama, Iwao; Honda, Mitsunori; Hirao, Norie*; Deng, J.; Narita, Ayumi

Journal of Physics; Conference Series, 100, p.012015_1 - 012015_4, 2008/00

https://doi.org/10.1088/1742-6596/100/1/012015

Chemical-state-selective mapping of micro-patterns for silicon compounds has been demonstrated using photoelectron emission microscopy excited by soft X-rays from synchrotron light source. The samples investigated were micro-patterns of silicon oxides, silicon nitrides, and organic silicon compounds. By scanning the X-ray energy around the Si -edge, we succeeded in observing the sub-micron images depending on the valence states. When we annealed the sample, the lateral diffusion was observed from 700C. During the annealing, however, no intermediate valence states were observed at the Si-SiO interfaces. It was elucidated that the diffusion of oxygen induced the sudden changes of the Si valence states from Si to Si without any intermediate valence states. The results for the chemical-state-selective mappings and lateral diffusions are also presented for organic silicon compounds.

Orientation of thin films synthesized from silicon phthalocyanine dichloride on a highly oriented pyrolytic graphite investigated using near edge X-ray absorption fine structure

Deng, J.; Sekiguchi, Tetsuhiro; Baba, Yuji; Hirao, Norie*; Honda, Mitsunori

Japanese Journal of Applied Physics, Part 1, 46(2), p.770 - 773, 2007/02

https://doi.org/10.1143/JJAP.46.770

Molecular orientation of thin films of silicon phthalocyanine (SiPc) compounds on highly oriented pyrolytic graphite (HOPG) was investigated by near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). The films were prepared by a casting method using solution of SiPc dichloride. XPS results showed that the chlorine atoms in SiPc dichloride were substituted by oxygen atoms when the film was heated in the air. The orientation of the molecules with respect to the substrate plane was investigated by the polarization dependences of the Si edge NEXAFS spectra. For the sample heated in the air, two clear peaks appeared in the NEXAFS spectra at around 1847.2 and 1852.4 eV, which were assigned to the resonant excitation form Si 1s to * orbitals around the Si-N bonds and those around the Si-O bonds, respectively. The intensities of the resonance peaks showed strong polarization dependence. A quantitative analysis of the polarization dependences revealed that the Si-N bond was lying down while the Si-O bond was out of the plane.

Chemisorption of amino acid monolayer on gold substrate

Honda, Mitsunori; Baba, Yuji; Hirao, Norie*; Deng, J.; Sekiguchi, Tetsuhiro

no journal, , 

no abstracts in English

Molecular orientation of organic thin films studied using synchrotron radiation and photoelectron emission spectroscopy

Sekiguchi, Tetsuhiro; Baba, Yuji; Shimoyama, Iwao; Hirao, Norie; Honda, Mitsunori; Deng, J.*

no journal, , 

For the purpose of better understanding of orientation mechanism of organic semiconductors, we are developing an instrument to measure nano-NEXAFS (near-edge X-ray absorption fine structure) with photoelectron emission microscopy (PEEM). The PEEM column can be rotated along the axis of synchrotron beam, so that one can measure polarization-angle dependences of NEXAFS spectra of each domain. According, it is possible to measure direction of chemical-bonds in nanometer scale. Resonant peaks observed in X-ray absorption spectra have been interpreted in terms of molecular orbital calculations based on equivalent core (Z+1) approximation model. The theoretical simulations sufficiently reproduced the experimental spectra. We have also investigated the effect of ligands by measuring NEXAFS spectra of SiPcXY and SiNaphXY (X, Y = CH, OH, Cl), where Naph refers to as naphthalocyanine. Origins of the observed absorption bands were reasonably understood.

Molecular orientation of Si- phthalocyanine organic semiconductor thin films revealed using synchrotron radiation

Sekiguchi, Tetsuhiro; Baba, Yuji; Shimoyama, Iwao; Honda, Mitsunori*; Hirao, Norie; Deng, J.*

no journal, , 

It has been highly demanded to synthesize thin films of organic semiconductors with high carrier density as much as possible. Recently, it has been reported that the high degrees of orientation plays a important role in the enhancement of the carrier density. One of possible ways can be proposed that oriented films be prepared as a result of polymerization of semiconductor monomers. In the present study, we have prepared thin films of silicon phthalocyanine dichloride, organic semiconductor, in organic solution on graphite substrates and the oriented thin films through hydrolysis and dehydration polymerization by annealing. We have investigated the atomic configuration and orientation of the films using synchrotron-based X-ray absorption spectroscopy, angle-depended NEXAFS and XPS. We have found that the polymerization results in the formation of one-dimensionally ordered molecular stacks which are oriented along one direction at the surface.

Thermal reaction and orientation of silicon-phthalocyanine dichloride thin films on metal surfaces

Sekiguchi, Tetsuhiro; Baba, Yuji; Shimoyama, Iwao; Hirao, Norie; Honda, Mitsunori; Deng, J.*

no journal, , 

The orientation of organic semiconducting molecules on surfaces plays a crucial role in improving the properties of electronic devices. We prepared thin films of silicon phthalocyanine dichroride (SiPcCl) on graphite and copper by spin-cast followed by heating to 350 C at ambient condition. We investigate the molecular orientation effect of the films using angle-dependent near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy. Si 1s NEXAFS spectra for the thin films after annealing are very different each other, indicating that the corresponding reactions thus the structures of final products are different. The structures of the reaction products are estimated with the help of ab initio molecular orbital calculations that fit the observed NEXAFS spectra. Moreover, for the purpose of better understanding of orientation mechanism, we develop an instrument to measure the dependence of linear-polarization angles of the VUV light on PEEM images of the thin film surface.