发布求助
文献互助 智能选刊 最新文献

Journal of Experimental and Theoretical Physics最新文献

筛选
英文 中文
Bragg Resonances in a Multiferroic Double-Nonlinearity Crystal 多铁性双非线性晶体中的布拉格共振
IF 1.1 4区 物理与天体物理
Journal of Experimental and Theoretical Physics Pub Date : 2023-12-10 DOI: 10.1134/s1063776123100060
M. A. Morozova, O. V. Matveev, D. V. Romanenko, S. A. Nikitov
{"title":"Bragg Resonances in a Multiferroic Double-Nonlinearity Crystal","authors":"M. A. Morozova, O. V. Matveev, D. V. Romanenko, S. A. Nikitov","doi":"10.1134/s1063776123100060","DOIUrl":"https://doi.org/10.1134/s1063776123100060","url":null,"abstract":"<h3 data-test=\"abstract-sub-heading\">Abstract</h3><p>The influence of double (electrical and magnetic) nonlinearity on the Bragg resonances of the hybrid electromagnetic-spin waves in a multiferroic crystal has been theoretically and experimentally revealed. The multiferroic crystal consists of an yttrium iron garnet layer with a periodic system of grooves on the surface and a ferroelectric strontium–barium titanate layer. A dispersion relation for the hybrid waves is obtained, and the mechanism of formation of main and hybrid band gaps, namely, suppression bands, is revealed. It is shown that taking into account the magnetic nonlinearity leads to frequency rearrangement of both band gaps and taking into account the electrical nonlinearity leads to frequency rearrangement of only the hybrid band gap. In the general case, the effects of the electrical and magnetic nonlinearities on the hybrid band gap can be compensated.</p>","PeriodicalId":629,"journal":{"name":"Journal of Experimental and Theoretical Physics","volume":null,"pages":null},"PeriodicalIF":1.1,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138562143","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Antiferromagnetic Excitonic Insulator 反铁磁励磁绝缘体
IF 1.1 4区 物理与天体物理
Journal of Experimental and Theoretical Physics Pub Date : 2023-12-10 DOI: 10.1134/s1063776123100138
V. V. Val’kov
{"title":"Antiferromagnetic Excitonic Insulator","authors":"V. V. Val’kov","doi":"10.1134/s1063776123100138","DOIUrl":"https://doi.org/10.1134/s1063776123100138","url":null,"abstract":"<h3 data-test=\"abstract-sub-heading\">Abstract</h3><p>The effective the two-band Hamiltonian is obtained for iridium oxides with account for strong electron correlations (SEC) and the spin–orbit interaction. The intraatomic electron correlations in iridium ions induce the formation of Hubbard fermions (HF) filling the states in the valence band. Another consequence of SEC is associated with the emergence of the antiferromagnetic (AFM) exchange interaction between HF in accordance with the Anderson mechanism. As a result, a long-range antiferromagnetic order is established in the system, and in the conditions of band overlapping, the intersite Coulomb interaction induces a phase transition to the excitonic insulator (EI) state with a long-range AFM order. The system of integral self-consistent equations, the solution to which determines the excitonic order parameter components Δ<sub><i>i</i>,</sub> <sub><i>j</i></sub>(<i>k</i>), sublattice magnetization <i>M</i>, Hubbard fermion concentration <i>n</i><sub><i>d</i></sub>, and chemical potential μ, is obtained using the atomic representation, the method of two-time temperature Green’s functions, and the Zwanzig–Mori projection technique. The symmetry classification of AFM EI phases is performed, and it is shown that in the nearest neighbor approximation, state Δ<sub><i>i</i>,</sub> <sub><i>j</i></sub>(<i>k</i>) with the <i>s</i>-type symmetry corresponds to the ground state, while the phases with the <i>d</i>- and <i>p</i>-symmetries are metastable.</p>","PeriodicalId":629,"journal":{"name":"Journal of Experimental and Theoretical Physics","volume":null,"pages":null},"PeriodicalIF":1.1,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138569798","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Optical Linear Dichroism in the ab-Plane of NdFe3(BO3)4 Ferroborate NdFe3(BO3)4 铁硼酸盐 ab 平面上的光学线性二色性
IF 1.1 4区 物理与天体物理
Journal of Experimental and Theoretical Physics Pub Date : 2023-12-10 DOI: 10.1134/s1063776123100011
K. N. Boldyrev, M. Diab, I. A. Gudim, M. N. Popova
{"title":"Optical Linear Dichroism in the ab-Plane of NdFe3(BO3)4 Ferroborate","authors":"K. N. Boldyrev, M. Diab, I. A. Gudim, M. N. Popova","doi":"10.1134/s1063776123100011","DOIUrl":"https://doi.org/10.1134/s1063776123100011","url":null,"abstract":"<h3 data-test=\"abstract-sub-heading\">Abstract</h3><p>The high-resolution optical absorption spectra of NdFe<sub>3</sub>(BO<sub>3</sub>)<sub>4</sub> single crystals have been recorded at temperatures from 4 to 40 K in the IR range of <i>f</i>–<i>f</i> transitions in a Nd<sup>3+</sup> ion. Light linearly polarized at different angles to the <i>C</i><sub>2</sub> axes in the basal plane has been passed along the trigonal <i>C</i><sub>3</sub> axis. Below the temperature of magnetic moment ordering into a collinear antiferromagnetic structure (<i>T</i><sub>N</sub> ≈ 30 K), dichroism, that is, the absorption versus polarization dependence, arises. The temperature and angular dependences of dichroism indicate that the magnetic moments of iron are directed along the <i>C</i><sub>2</sub> axes up to about 17 K, the number of domains with variously directed <i>C</i><sub>2</sub> axes being different. The mechanism of linear dichroism has been discussed. Below 17 K, a smooth transition to the helicoidal magnetic phase has been observed, with the collinear phase coexisting with the helicoidal one. Data presented in this article contradict the earlier concept of magnetic moments fluctuating in the low-temperature phase near the <i>C</i><sub>2</sub> axis within the ±10° interval.</p>","PeriodicalId":629,"journal":{"name":"Journal of Experimental and Theoretical Physics","volume":null,"pages":null},"PeriodicalIF":1.1,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138569413","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Magnetic Properties of the Double Perovskite Sr2CrNbO6 双包晶石 Sr2CrNbO6 的磁性能
IF 1.1 4区 物理与天体物理
Journal of Experimental and Theoretical Physics Pub Date : 2023-12-10 DOI: 10.1134/s1063776123100096
D. V. Popov, R. G. Batulin, M. A. Cherosov, I. V. Yatsyk, T. I. Chupakhina, Yu. A. Deeva, I. I. Fazlizhanov, R. M. Eremina, T. Maiti
{"title":"Magnetic Properties of the Double Perovskite Sr2CrNbO6","authors":"D. V. Popov, R. G. Batulin, M. A. Cherosov, I. V. Yatsyk, T. I. Chupakhina, Yu. A. Deeva, I. I. Fazlizhanov, R. M. Eremina, T. Maiti","doi":"10.1134/s1063776123100096","DOIUrl":"https://doi.org/10.1134/s1063776123100096","url":null,"abstract":"<h3 data-test=\"abstract-sub-heading\">Abstract</h3><p>A double perovskite Sr<sub>2</sub>CrNbO<sub>6</sub> powder compound was studied by using X-ray diffraction, AC and DC magnetization, and ESR measurements. Two transitions in antiferromagnetically ordered regimes were observed through magnetization measurements at <i>T</i> = 5 and 2 K and were confirmed by the linear dependence of the magnetization on the applied magnetic field at these temperatures, approximations of the temperature dependence of the ESR linewidth, and AC magnetization. The zero field cooling curve was approximated by Bonner–Fisher law for quasi one dimensional chain with the exchange integral <i>J</i>/<i>k</i><sub>B</sub> = 1 K between chromium spins. An approximation of linear part of magnetic susceptibility temperature dependence was performed using Curie–Weiss law. To describe the obtained effective moment μ<sub>eff</sub> = 3.577μ<sub>B</sub>, the presence Cr<sup>3+</sup> and Cr<sup>4+</sup> ions is estimated at a respective ratio about 0.8 : 0.2.</p>","PeriodicalId":629,"journal":{"name":"Journal of Experimental and Theoretical Physics","volume":null,"pages":null},"PeriodicalIF":1.1,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138569540","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
95th Anniversary of the Birthday of Ludmila Prozorova 柳德米拉-普罗佐洛娃（Ludmila Prozorova）诞辰 95 周年
IF 1.1 4区 物理与天体物理
Journal of Experimental and Theoretical Physics Pub Date : 2023-12-10 DOI: 10.1134/s1063776123100199
{"title":"95th Anniversary of the Birthday of Ludmila Prozorova","authors":"","doi":"10.1134/s1063776123100199","DOIUrl":"https://doi.org/10.1134/s1063776123100199","url":null,"abstract":"","PeriodicalId":629,"journal":{"name":"Journal of Experimental and Theoretical Physics","volume":null,"pages":null},"PeriodicalIF":1.1,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138569344","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Structural Disorder, Specific Heat, and Magnetic Transitions in Cu2FeBO5 Cu2FeBO5 中的结构紊乱、比热和磁性转变
IF 1.1 4区 物理与天体物理
Journal of Experimental and Theoretical Physics Pub Date : 2023-12-10 DOI: 10.1134/s1063776123100175
Yu. S. Gokhfeld, N. V. Kazak, N. A. Bel’skaya, M. S. Molokeev, I. A. Gudim, O. A. Kondratiev, E. V. Eremin, Yu. V. Knyazev, D. A. Velikanov, S. G. Ovchinnikov
{"title":"Structural Disorder, Specific Heat, and Magnetic Transitions in Cu2FeBO5","authors":"Yu. S. Gokhfeld, N. V. Kazak, N. A. Bel’skaya, M. S. Molokeev, I. A. Gudim, O. A. Kondratiev, E. V. Eremin, Yu. V. Knyazev, D. A. Velikanov, S. G. Ovchinnikov","doi":"10.1134/s1063776123100175","DOIUrl":"https://doi.org/10.1134/s1063776123100175","url":null,"abstract":"<h3 data-test=\"abstract-sub-heading\">Abstract</h3><p>Cu<sub>2</sub>FeBO<sub>5</sub> ludwigite single crystals have been grown from a solution–melt by spontaneous crystallization. Using the X-ray diffraction method, the crystal structure has been resolved in detail. Cations in sites M2, M3, and M4 have turned out to be structurally disordered. It has been found that oxygen atoms are disordered in one of five nonequivalent sites (O4). As can be seen from Mössbauer spectroscopy data, Fe<sup>3+</sup> ions occupy four nonequivalent sites with different distortions of coordination octahedra. In the temperature range 40 K ≤ <i>T</i> ≤ 300 K, the spectra represent a superposition of quadrupole doublets. Static susceptibility measurements have revealed two magnetic features at <i>T</i><sub>1</sub> = 35 K and <i>T</i><sub>2</sub> = 20 K and spin-glass effects. Specific heat measurements in the interval 4–300 K have not discovered magnetic-transition-related anomalies.</p>","PeriodicalId":629,"journal":{"name":"Journal of Experimental and Theoretical Physics","volume":null,"pages":null},"PeriodicalIF":1.1,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138569539","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Magnetic Resonance in the Quasi-2D Square Lattice Easy-Plane Antiferromagnet Ba2MnGe2O7 准二维方格易平面反铁磁体 Ba2MnGe2O7 中的磁共振
IF 1.1 4区 物理与天体物理
Journal of Experimental and Theoretical Physics Pub Date : 2023-12-10 DOI: 10.1134/s1063776123100163
V. N. Glazkov, Yu. V. Krasnikova, I. K. Rodygina, M. Hemmida, M. Hirrle, H.-A. Krug von Nidda, T. Masuda
{"title":"Magnetic Resonance in the Quasi-2D Square Lattice Easy-Plane Antiferromagnet Ba2MnGe2O7","authors":"V. N. Glazkov, Yu. V. Krasnikova, I. K. Rodygina, M. Hemmida, M. Hirrle, H.-A. Krug von Nidda, T. Masuda","doi":"10.1134/s1063776123100163","DOIUrl":"https://doi.org/10.1134/s1063776123100163","url":null,"abstract":"<h3 data-test=\"abstract-sub-heading\">Abstract</h3><p>We report results of a multi-frequency (0.8–60 GHz) electron spin resonance study of the spin dynamics in the quasi-2D square lattice antiferromagnet Ba<sub>2</sub>MnGe<sub>2</sub>O<sub>7</sub> both in antiferromagnetically ordered and paramagnetic phases. We directly observe two zero-field gaps in the excitation spectrum of the ordered phase, the larger one being due to easy-plane anisotropy, and the smaller one indicates the presence of fourth-order in-plane anisotropy probably related to the multiferroic properties of this compound. We observe effects of hyperfine interaction on the electron spin resonance spectra in the antiferromagnetically ordered state, which turns out to be comparable with in-plane anisotropy. The hyperfine field strength is found from the observed low-temperature electron spin resonance data. The spin dynamics of the paramagnetic phase is characterized by strong broadening of the ESR absorption line, which can be ascribed to the vortex dynamics of a 2D magnet.</p>","PeriodicalId":629,"journal":{"name":"Journal of Experimental and Theoretical Physics","volume":null,"pages":null},"PeriodicalIF":1.1,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138569541","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Role of Vacancies in the Yao–Lee Model 空缺职位在姚李模型中的作用
IF 1.1 4区 物理与天体物理
Journal of Experimental and Theoretical Physics Pub Date : 2023-12-10 DOI: 10.1134/s1063776123100084
V. A. Polyakov, N. B. Perkins
{"title":"Role of Vacancies in the Yao–Lee Model","authors":"V. A. Polyakov, N. B. Perkins","doi":"10.1134/s1063776123100084","DOIUrl":"https://doi.org/10.1134/s1063776123100084","url":null,"abstract":"<h3 data-test=\"abstract-sub-heading\">Abstract</h3><p>We consider the effect of vacancies on the low-energy excitation spectrum of a quantum spin liquid realized in the exactly solvable Yao–Lee model [H. Yao and D.-H. Lee, Phys. Rev. Lett. <b>107</b>, 087205 (2011)]. Physically, vacancies can appear for different reasons (e.g., because of zero magnetic moments on the lattice, or the presence of nonmagnetic impurities, or a random reduction of local bonds of magnetic moments with the remaining lattice). It is shown numerically that the finite density of random vacancies in this model leads to the accumulation of states near zero energy, which can be detected from the change of the behavior of heat capacity at low temperatures. Moreover, it is shown that the low-energy modes are localized more strongly than remaining eigenmodes. This effect is illustrated using the inverse participation ratio (IPR). In the case of time reversal symmetry breaking (e.g., due to the presence of a magnetic field), a gap is opened in the fermion spectrum of the model, and vacancy-induced localized states appear. The energies of these states depend on the structure of the interactions responsible for the time inversion symmetry breaking.</p>","PeriodicalId":629,"journal":{"name":"Journal of Experimental and Theoretical Physics","volume":null,"pages":null},"PeriodicalIF":1.1,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138569634","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Spin-Flop Transition in Spiral Magnets 螺旋磁体中的自旋-翻转转变
IF 1.1 4区 物理与天体物理
Journal of Experimental and Theoretical Physics Pub Date : 2023-12-10 DOI: 10.1134/s1063776123100047
S. K. Gotovko, V. I. Marchenko
{"title":"Spin-Flop Transition in Spiral Magnets","authors":"S. K. Gotovko, V. I. Marchenko","doi":"10.1134/s1063776123100047","DOIUrl":"https://doi.org/10.1134/s1063776123100047","url":null,"abstract":"<h3 data-test=\"abstract-sub-heading\">Abstract</h3><p>It has been shown that at spin-flop transition in spiral magnets LiCuVO<sub>4</sub>, LiCu<sub>2</sub>O<sub>2</sub>, and CuCrO<sub>2</sub>, Goldstone mode looses its stability.</p>","PeriodicalId":629,"journal":{"name":"Journal of Experimental and Theoretical Physics","volume":null,"pages":null},"PeriodicalIF":1.1,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138569351","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Dispersion Relation in Amorphous Ferromagnets 非晶铁磁体中的弥散关系
IF 1.1 4区 物理与天体物理
Journal of Experimental and Theoretical Physics Pub Date : 2023-12-10 DOI: 10.1134/s1063776123100059
S. V. Grigoriev, L. A. Azarova, K. A. Pshenichnyi, O. I. Utesov
{"title":"Dispersion Relation in Amorphous Ferromagnets","authors":"S. V. Grigoriev, L. A. Azarova, K. A. Pshenichnyi, O. I. Utesov","doi":"10.1134/s1063776123100059","DOIUrl":"https://doi.org/10.1134/s1063776123100059","url":null,"abstract":"<h3 data-test=\"abstract-sub-heading\">Abstract</h3><p>Dispersion of spin waves in the amorphous ferromagnetic alloy Fe<sub>48</sub>Ni<sub>34</sub>P<sub>18</sub> can be described within the model of a ferromagnet with random anisotropy: <span>(epsilon )</span>(<i>q</i>) = <i>Aq</i><sup>2</sup> + <i>g</i>μ<sub>B</sub><i>H</i> + δω(<i>q</i>), where δω(<i>q</i>) is an additional term linear in |<i>q</i>|. The method of small-angle scattering of polarized neutrons is used to prove the importance of the additional term δω(<i>q</i>) in dispersion. The measurements are carried out for different values of the external magnetic field <i>H</i> and neutron wavelength λ. The scattering map of neutrons represents a circle centered at the point <i>q</i> = 0. The stiffness <i>A</i> of spin waves is derived directly from the λ-dependence of the radius of this circle. The spin-wave stiffness <i>A</i> of the amorphous alloy weakly decreases from 140 to 110 meV Å<sup>2</sup> as temperature increases from 50 to 300 K. The field dependence of the radius demonstrates the presence of an additional term δω(<i>q</i>) in the form of an energy gap that is almost independent of field and temperature. The value of the additional term is Δ = 0.015 ± 0.002 meV.</p>","PeriodicalId":629,"journal":{"name":"Journal of Experimental and Theoretical Physics","volume":null,"pages":null},"PeriodicalIF":1.1,"publicationDate":"2023-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138569475","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
首页 上一页 下一页 尾页
0
×
引用
GB/T 7714-2015
复制
MLA
复制
APA
复制
导出至
BibTeX EndNote RefMan NoteFirst NoteExpress
×
提示
您的信息不完整，为了账户安全，请先补充。
现在去补充
×
提示
您因"违规操作"
具体请查看互助需知
我知道了
×
提示
现在去查看 取消
×
提示
确定
请完成安全验证×
相关产品
×
本文献相关产品
联系我们：info@booksci.cn Book学术提供免费学术资源搜索服务，方便国内外学者检索中英文文献。致力于提供最便捷和优质的服务体验。 Copyright © 2023 布克学术 All rights reserved.
京ICP备2023020795号-1
ghs 京公网安备 11010802042870号
Book学术文献互助
Book学术文献互助群
群 号：481959085
Book学术官方微信