查看“︁磁振子”︁的源代码

{{NoteTA|G1=Physics}}
{{Condensed matter physics|expanded=Quasiparticles}}
'''磁振子'''是晶格中電子[[自旋]]結構[[準粒子|集體激發]]的[[準粒子]]。在量子力學的等效波圖中，磁振子可以被看作是[[量子化|量化]]的{{tsl|en|spin wave|自旋波}}，也就是[[磁性]][[有序體]]的動態[[本徵激發]]。磁振子攜帶著固定量的[[能量]]和{{tsl|en|Crystal momentum|晶格動量}}，是自旋1的準粒子，並且服從玻色子的行為。

==歷史背景==
為了解釋在[[鐵磁體]]中[[自發磁化]]的減少，磁振子的概念在1930年由[[費利克斯·布洛赫]]（Felix Bloch）引入。在[[絕對零度]]溫度下，[[海森堡模型#一維海森堡模型|海森堡鐵磁體]]達到最低能量的狀態，其中所有原子自旋指向相同的方向，因此[[磁矩]]也指向相同的方向。隨著溫度升高，越來越多的自旋從原先對準的方向中隨機偏離，使內部能量增加並減小淨磁化強度。如果將零溫度下的完全磁化狀態視為鐵磁體的{{tsl|en|vacuum state|真空狀態}}，則具有少量不對準自旋的低溫狀態可以被視為磁振子的氣體。每個磁振子使沿著磁化方向的總自旋減小ħ，並使該方向的磁化強度減小γħ，其中γ是[[旋磁比]]。這造成了自發磁化與溫度相關的布洛赫定律：

:<math>M(T) = M_0 (1 - (T/T_C)^{3/2})</math>

磁振子的定量理論，也就是量化自旋波，由[[西奧多·霍爾斯坦]]（Theodore Holstein），{{tsl|en|Henry Primakoff|亨利·普里馬科夫}}（Henry Primakoff）和[[弗里曼·戴森]]（Freeman Dyson）做了進一步開發。使用[[正則量子化]]方法，他們發現磁振子的表現像有微弱相互作用的準粒子並遵守[[玻色–愛因斯坦統計]]，就像玻色子一樣。 相關的綜合探討可以在[[查爾斯·基泰爾]]（Charles Kittel）的基礎固態物理學教科書或Van Kranendonk和[[約翰·凡扶累克]]（J. H. VAN VLECK）的早期綜述文章<ref>{{Cite journal|title = Spin Waves|date =  1 January 1958|journal = Rev. Mod. Phys. |doi = 10.1103/RevModPhys.30.1|volume=30 |pages=1--23}}</ref>中找到。

在1957年[[伯特倫·布羅克豪斯]]通過在[[鐵氧體]]中的非彈性{{tsl|en|neutron scattering|中子散射}}直接測得磁振子。 從那時起，在[[鐵磁體]]，[[亞鐵磁性|亞鐵磁體]]和[[反鐵磁體]]中也都檢測到了磁振子。

磁振子遵守[[玻色–愛因斯坦統計]]的事實在20世紀60年代到80年代期間進行的光散射實驗得到證實。古典理論預測[[斯托克斯位移|斯托克斯線]]和反斯托克斯線的強度相等。然而，散射實驗結果顯示，如果磁能能量相近於或小於熱能，或 <math>\hbar \omega  < k_B T</math>，則斯托克斯線變得比反斯托克斯線更強烈，和從玻色–愛因斯坦統計中計算出來的結果吻合。1999年[[東京工業大學]]的二国徹郎等人在低溫的反鐵磁物質中證明了磁振子的[[玻色–愛因斯坦凝聚]]<ref>{{Cite journal|url = |title = Bose-Einstein Condensation of Dilute Magnons in TlCuCl<sub>3</sub>|date = 19 June 2000|journal = Phys. Rev. Lett. |doi = 10.1103/PhysRevLett.84.5868|volume=84|issue = 25 |pages=5868--5871}}</ref>，而2006年3月[[明斯特大學]]應用物理研究所的S. O. Demokritov等人則是在室溫的[[鐵氧體]]中證明了磁振子的玻色–愛因斯坦凝聚<ref>{{Cite journal|url =https://archive.org/details/sim_nature-uk_2006-09-28_443_7110/page/430|title = Bose–Einstein condensation of quasi-equilibrium magnons at room temperature under pumping|last = |first = |date = 28 September 2006|journal = Nature |doi = 10.1038/nature05117|pmid = 17006509|access-date = |bibcode = 2006Natur.443..430D |volume=443 |pages=430–433}}</ref> 。2013年8月內藤忠一等人報導了通過[[表面電漿共振]]可產生自旋電流<ref>{{Cite journal|title = Generation of spin currents by surface plasmon resonance|url = http://www.nature.com/ncomms/2015/150108/ncomms6910/full/ncomms6910.html|journal = Nature Communications|date = January 8, 2015|pmc = 4354158|pmid = 25569821|volume = 6|doi = 10.1038/ncomms6910|first = K.|last = Uchida|first2 = H.|last2 = Adachi|first3 = D.|last3 = Kikuchi|first4 = S.|last4 = Ito|first5 = Z.|last5 = Qiu|first6 = S.|last6 = Maekawa|first7 = E.|last7 = Saitoh|arxiv = 1308.3532|bibcode = 2015NatCo...6E5910U|pages = 5910|access-date = 2017-01-13|archive-date = 2016-08-20|archive-url = https://web.archive.org/web/20160820152534/http://www.nature.com/ncomms/2015/150108/ncomms6910/full/ncomms6910.html|dead-url = no}}</ref>。

== 特性 ==
磁振子的行為可以透過各種[[散射]]技術來研究。 磁振子的行為就如同沒有化學勢的[[玻色氣體]]。微波泵浦可用來激發自旋波並產生額外的非平衡磁振子，非平衡磁振子熱化後會形成[[聲子]]。磁振子在臨界密度下會形成[[玻色–爱因斯坦凝聚|冷凝物]]，並發射單色的微波。這種微波源的狀態可以由改變外加磁場強度來調整。

== 參閱 ==
*{{tsl|en|Magnonics|磁力學}}
*{{tsl|en|Holstein-Primakoff transformation|荷斯坦-普里馬科夫轉換}}
*{{tsl|en|Spin pumping|自旋泵浦效应}}
*{{tsl|en|Spin Hall effect|自旋霍爾效應}}

==註釋==
{{Reflist}}

== 參考文獻 ==
* C. Kittel, ''Introduction to Solid State Physics'', 7th edition (Wiley, 1995).  ISBN 0-471-11181-3.
* {{cite journal | last1 = Bloch | first1 = F. | year = 1930 | title =  Zur Theorie des Ferromagnetismus| doi = 10.1007/bf01339661 | journal = Z. Phys. | volume = 61 | issue = | pages = 206–219 |bibcode = 1930ZPhy...61..206B }}
* {{cite journal | last1 = Holstein | first1 = T. | last2 = Primakoff | first2 = H. | year = 1940 | title =  Field Dependence of the Intrinsic Domain Magnetization of a Ferromagnet| url =https://archive.org/details/sim_physical-review_1940-12-15_58_12/page/n86| journal = Phys. Rev. | volume = 58 | issue = | pages = 1098–1113 | doi = 10.1103/PhysRev.58.1098 |bibcode = 1940PhRv...58.1098H }}
* {{cite journal | last1 = Dyson | first1 = F. J. | year = 1956 | title =  General Theory of Spin-Wave Interactions| url =https://archive.org/details/sim_physical-review_1956-06-01_102_5/page/n29| journal = Phys. Rev. | volume = 102 | issue = | pages = 1217–1230 | doi = 10.1103/PhysRev.102.1217 |bibcode = 1956PhRv..102.1217D }}
* {{cite journal | last1 = Brockhouse | first1 = B. N. | year = 1957 | title =  Scattering of Neutrons by Spin Waves in Magnetite| url =https://archive.org/details/sim_physical-review_1957-06-01_106_5/page/n21| journal = Phys. Rev. | volume = 106 | issue = | pages = 859–864 | doi = 10.1103/PhysRev.106.859 |bibcode = 1957PhRv..106..859B }}
* {{cite journal | last1 = Kranendonk | first1 = J. Van | last2 = Vleck | first2 = J. H. Van | year = 1958 | title =  Spin Waves| url = | journal = Rev. Mod. Phys. | volume = 30 | issue = | pages = 1–23 | doi = 10.1103/RevModPhys.30.1 | bibcode=1958RvMP...30....1V}}
* {{cite journal | last1 = Nikuni | first1 = T. | last2 = Oshikawa | first2 = M. | last3 = Oosawa | first3 = A. | last4 = Tanaka | first4 = H. | year = 1999 | title =  Bose-Einstein Condensation of Dilute Magnons in     TlCuCl   3| url = | journal = Phys. Rev. Lett. | volume = 84 | issue = | pages = 5868–5871 | doi = 10.1103/PhysRevLett.84.5868 | bibcode=2000PhRvL..84.5868N | pmid=10991075|arxiv = cond-mat/9908118 }}
* {{cite journal | last1 = Demokritov | first1 = S. O. | last2 = Demidov | first2 = V. E. | last3 = Dzyapko | first3 = O. | last4 = Melkov | first4 = G. A. | last5 = Serga | first5 = A. A. | last6 = Hillebrands | first6 = B. | last7 = Slavin | first7 = A. N. | year = 2006 | title =  Bose-Einstein condensation of quasi-equilibrium magnons at room temperature under pumping| url = | journal = Nature | volume = 443 | issue = | pages = 430–3 | doi = 10.1038/nature05117 | pmid=17006509|bibcode = 2006Natur.443..430D }}
* P. Schewe and B. Stein, Physics News Update '''746''', 2 (2005).  [http://www.aip.org/pnu/2005/split/746-2.html online] {{Wayback|url=http://www.aip.org/pnu/2005/split/746-2.html |date=20130410042631 }}
* {{cite journal | last1 = Kimel | first1 = A.V. | authorlink3 = Theo Rasing | last2 = Kirilyuk | first2 = A. | last3 = Rasing | first3 = T.H. | year = 2007 | title = Femtosecond opto-magnetism: ultrafast laser manipulation of magnetic materials | url = http://www3.interscience.wiley.com/journal/116840805/abstract | journal = Laser & Photon Rev. | volume = 1 | issue = 3 | pages = 275–287 | doi = 10.1002/lpor.200710022 | access-date = 2017-01-13 | archive-date = 2013-01-05 | archive-url = https://archive.today/20130105080519/http://www3.interscience.wiley.com/journal/116840805/abstract | dead-url = yes }}

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[[Category:準粒子]]