Quantum Materials Dynamics Group

We are a theoretical group studying the dynamics of quantum materials:
1) Magnetism: quasiparticle chiral interactions; nonlinear magnon transport.
2) Spintronics: many-body spin dynamics; various spin phenomena including spin Hall, spin pumping, and Seebeck effects.
3) Unconventional superconductivity: Keldysh formalism for ultrafast optical engineering; superconducting mechanism in moire system.
4) Semiconductor optics: valleytronics in monolayer TMDs; optical engineering of quantum states.

Quantum Materials Dynamics Group

News and Blog

Our joint experimental and theoretical paper on Science Advances demonstrates the chirality of microwave evanescent fields that can unidirectionally pump spin waves with a focusing due to the anisotropic dispersion
Our joint experimental and theoretical paper on Science Advances demonstrates the chirality of microwave evanescent fields that can unidirectionally pump spin waves with a focusing due to the anisotropic dispersion

Paper: https://advances.sciencemag.org/content/6/46/eabd3556

Magnets, probably the most conventional materials that has attracted the attention of physicists for thousands of years, are important components in every computer for logic operations. The spins in the magnets are coupled via the exchange interaction, which form waves when they move, which can avoid Joule heating in insulators and can propagate very far. More attractively, the spins can only precess anticlockwise around the magnetic field, known as chirality, which promise lots of functionalities such as unidirectional spin transport, chiral spin Seebeck effect, non-contact spin pumping, magnonic non-Hermitian skin effect, spin blockage/trap with perfect energy transfer between magnets, and magnon/ phonon/microwave photon/electron diode effects, as revealed by a theoretical team in MPSD, Delft University of technology and Tohoku university in several theoretical papers on Physical Review Letters (Yu et al., Chiral Pumping of Spin Waves, Magnon Accumulation in Chirally Coupled Magnets, Noncontact Spin Pumping by Microwave Evanescent Fields, Unidirectional Pumping of Phonons by Magnetization Dynamics). However, directly imaging the spin waves and their chirality are ever challenging since they are inertial to the light and cannot be easily controlled by conventional electric method.

The theoretical team now collaborate with Van der Sar lab in Delft University of technology to image the chirality of spin waves. Van der Sar lab introduces a platform for probing coherent spin waves based on magnetic resonance imaging with electron spins in diamond. A significant technique advance allows them to image the spin-wave packet in time-domain. Just like watching water waves in the lake by eyes, scientists now can directly watch how the spin waves move. These results are the most direct demonstration of spin-wave chirality that deepen our understanding of spin waves dynamics, which could help to design pure spin-wave based circuit with unidirectional transport in the future. The team’s work has been published in Science Advances recently.

The theoreticians reveal or clarify a large class of devices that support chiral interaction, viz., a nanomagnet can only couple with the waves propagating along one direction, among various quasiparticles including magnon, electrons and phonons. A key prediction in the chiral coupling rely on the dipolar field from the spin waves. Dipolar field can be imagined as lots of force lines that connect the north pole of a magnet to its south pole. Since the spins of magnons are precessing, they generate dynamical dipolar field that are propagating with the spin waves. In a magnetic film, the right-propagating spin waves can only generate the dipolar field at one side of the film, say above the film, the left-propagating spin waves can only dwell below the film. This property, however, is difficult to be probed directly since the dipolar field is very tiny and decays fast in hundreds of nanometers away from the film.

Spins in nitrogen-vacancy (NV) center of diamond, which is ever a good candidate as qubit for quantum computing, is very sensitive to the tiny magnetic field and can be put very close to the magnetic film that turns out to be very powerful to detect the dipolar field of the spin waves. Iacopo Bertelli et al. in Van der Sar lab use the NV center of diamonds to directly image spin-wave interference with phase resolution, revealing unidirectional, autofocused spin-wave patterns with frequency-controlled numerical apertures. To see how the spin waves move, they introduce external microwaves as reference that interferes with the dipolar field of spin waves, also a kind of microwaves, allowing them to see the standing-wave patterns that evolve with time. Surprisingly, the ultrasensitive NV center images only spin wave propagating in one direction and in half space, a key characteristic of the spin-wave chirality. Also, very intriguingly, the excited spin waves are focused at one point in the film, at which point the amplitude of spin waves are much larger than the other places. We can expect that the unidirectional, self-focused spin waves can transport information in a very efficient way. These observations are explained by the theoreticians in MPSD and Tohoku University in terms of chiral spin-wave excitation and dipolar-field coupling to the sensor spins.

巴黎假日(1)
巴黎假日(1)

By 姜雨含

十七岁梦想能去巴黎,那时晴天下雨,每天都好天气……三十岁纯真岁月远离,春天和秋天都走的那么急,年少的心事现在全已忘记,悄然又无息我却来到巴黎。

每一个少女心中应该都有一个巴黎梦,不知道从何时,或许从一部电影,一首歌,一张照片,一本书开始,巴黎就像一颗种子,悄悄种在了十七岁少女的心中。我也不落窠臼的爱着巴黎铁塔,塞纳河畔的咖啡厅,香榭丽舍大街的梧桐,巴黎圣母院的钟声,凡尔赛宫的镜廊,还有卢浮宫里的蒙拉丽莎。27岁后的这个盛夏,悄然无息我来到了梦想中的巴黎。 这是我们共同选择的欧洲自由行的第一站,对你来说巴黎是历史的,恢弘的,对我来说巴黎是浪漫的,圆梦的。我知道我必须要在卢浮宫前,凯旋门,铁塔下留下永远定格的照片,以此宣告我来到了巴黎。而对你来说,巴黎的历史在卢浮宫的珍宝中,在街头巨石垒起的一幢幢建筑中,在镂空的玫瑰花窗中,在墙壁上的每一个雕塑中……不同的旅行心态,一个时刻强调,仿佛打了鸡血一般必须要完成某种仪式般的任务,“快点,快点,我们要去下一个景点啦……快在这里给我拍张照片……我一定要去夏悠宫,网上都说了这是拍铁塔的最佳位置……”,我就是那个一路上叽叽喳喳,兴奋得不行,一天恨不得走遍每个经典的,追求仪式感的旅行者。而你就想在街头走走停停,恨不得待在卢浮宫里不出来,没有什么是必须要做的,也没有哪张照片是必须要留下的那个随性旅行者。所以我们的旅程会有小的争吵,虽然你都是完全听从我的安排,可是身体却很诚实的在反抗,第一天的暴走对于你这个能躺着绝不坐着,能坐着绝不站着的人来说的确很有挑战,脚上都走起了水泡。而兴奋的我却意犹未尽,最终只能妥协未能登顶凯旋门,一览巴黎。

清晨,在巴黎的第一缕晨光中,我们前往卢浮宫。未开馆,排队的长龙已经静候在广场。逛过中国的、荷兰的许多博物馆,卢浮宫却不能简单定义为法国博物馆,因为它是属于世界的,这里收藏着五大洲,各个文明的瑰宝,讲述的不只是法兰西的历史,还诉说着人类文明的历程。从古埃及到美索不达米亚平原的两河文明,从古希腊到古罗马,从非洲的古老图腾到美洲的玛雅文明……诉说着一个一个文明的辉煌和没落。卢浮宫就是一本世界历史教科书,那些曾经在教科书中讲述的历史,在这里迎面展开。汉谟拉比法典、狮身人面像、胜利女神、断臂的维纳斯、蒙拉丽莎的微笑,这些名字就足够让你惊叹。还有无数我们其实并不了解的故事,我实在是不懂艺术,也就不在这里班门弄斧,故装高雅。在逛博物馆时突然有点理解什么是“贵族”,这真不是用金钱能够买到的称号,这也不是权利的象征,这是一个家庭对文化,艺术,修养,知识的代代传承。我希望我的家庭留给孩子的是一颗颗脱离了低级趣味的种子,它能够长成对于社会,自然,和一切美好的,或者阴暗的事物的独立认知之树,它能够让你在命运的低谷和夹缝中依然开出绚烂之花,它让你既能承受伟大也能接受平庸。即使拿着贫民的工资,内心依然做一个骄傲的“贵族”吧。对于卢浮宫艺术作品的介绍我开设了一个新系列——卢浮宫300经典巨作,当时在纪念品商店购买的一本英文版的介绍书籍,打算花时间认真的翻译一下,不然就真是那个打卡游客啦。

下午天气炎热,出了卢浮宫我们在杜丽花园的树荫下,吃着冰淇淋享受着惬意的午后巴黎。最爱的就是在巴黎街头闲逛,夕阳下的歌剧院金碧辉煌,香榭丽舍大街车水马龙,人头攒动。我们一直沿着这条让人迷恋的浪漫之路向着凯旋门行进,当真实的到了这里后,发现原来它并没有什么无与伦比的景致,时尚的精品店也不敢踏入,走在这里又被拉回了现实世界,原来绚烂的物质文明还是需要那“庸俗”的金钱才能来享受。我们到达凯旋门时已经是晚上九点左右,炉火已经点燃,夏季的巴黎此时太阳还未完全落下,夕阳的余晖映照着凯旋门,熊熊炉火跳跃着,这座原本为了迎接拿破仑凯越的凯旋门并未能等来主人的胜利凯旋,但是它依然见证了无数法兰西斗士们为了名族荣誉和生存所付出的努力。君主专制下的巴黎恢宏奢华,这是尽全国之力发展起来的,这里是君王们的秀场,无数战利品,攫取的珍宝在这里陈列,可不知在这繁华巴黎的背后是否有无数底层法国人民以及被征服和剥削国家人民的叹息和呐喊。最终也是这样的奢华压垮了帝国,熊熊烈火烧毁了象征着君主专制的法国大皇宫。感谢宽容的人民并没有毁掉这一切,还留下了那些帝国曾经辉煌奢靡的记忆。巴黎的夜幕降临,一天的行程在凯旋门下结束。

最美的湖光山色(壹)——瑞法德5日游
最美的湖光山色(壹)——瑞法德5日游

By 姜雨含

终于在到荷兰4个多月后来了一次跨国旅行,从鹿特丹出发,穿过德意志、法兰西,深入到瑞士的湖光山色之中。对于瑞士的美,早有耳闻,这一次真正要踏上这个被无数人赞颂过的美丽国度,去看那最蓝的天空,最绿的草地,最清的湖水,内心是激动雀跃的。我们的大巴车一路沿着莱茵河向瑞士进发,中午抵达了德国的杜塞尔多夫,虽说是跨国旅行,可真是一点也没有出国的感觉,半天的功夫,在高速公路上就这么畅通无阻的到达了德国。对于德国的初映像果然和想象中的一致,没有太多风情,严谨略显古板,它的很多城市都是二战后重建的,因此看上去会少了些历史感。不过德国也有保留完好的历史古城,比如海德堡。下午继续赶路,我们畅通无阻的穿过了曾经的马其诺防线进入法国,在这里已经完全看不到曾经的壁垒战壕,看不到战争留下的任何痕迹,只有大片安静的葡萄园和农田。

傍晚我们抵达了法国阿尔萨斯省的首府斯特拉斯堡,欧洲历史上就是由一个个比较松散的城邦组成,很多城市的历史远远超过他们目前所属国家的历史,所以很多人心目中先是对于城市的认同感,然后才是对于国家的认同感。就像斯特拉斯堡,这是一座法语和德语通用的城市,它建城时还没有统一的法国和德国,由于它地处德法边境,所以在历史上被德国,法国分别统治占领过。阿尔萨斯人形成了他们独特的性格特征,兼具法国的浪漫情怀又融入了德意志民族的认真严谨,因此他们也有着一种优越感,认为自己比任何法国人,任何德国人都优秀,他们是阿尔萨斯人。这是我第一次走进法国,感受到法兰西的浪漫风情,相比德国我是更爱法国的。不论是建筑上,还是人文上,这里更加让人感觉放松和享受。法国人的确爱热闹,露天咖啡馆坐满了人,广场上流浪的艺人演奏着动人的旋律,整个城市沉浸在咖啡,美酒与音乐之中。

这里夏季天黑的特别晚,一般十点多才会完全黑下来,所以夜晚趁着夕阳的余晖我们游览了斯特拉斯堡大教堂,在小法兰西漫无目的的游荡,感受这浪漫的空气。斯特拉斯堡大教堂始建于12世纪,15世纪才建成,中途因为资金缺乏所以只建了一个塔,现在反而成了它的一大特色。教堂是典型的哥特式建筑,高逸、空灵,有大量的镂空玻璃窗和雕塑,整个建筑形体尖峭,姿态放纵,既华丽又清灵,既多姿又丰茂。在欧洲古代,教堂被认为是神、上帝的殿堂,而建的越高就更加方便神下来居住,所以欧洲哥特式教堂都是向上延伸,这样能够祈祷神常常降临。最近看《巴黎圣母院》时,当我读到雨果为圣母院教堂内的圣克里斯托弗的巨像被推倒而感到痛心时所说的这样一句话“那可是天下雕像中的佼佼者,正如天下大厅莫过于司法官大堂,天下钟楼莫过于斯特拉斯堡的尖塔一样”,我惊呼“天啦,斯特拉斯堡的尖塔我见过,我真真实实的见过,我到过雨果眼中最壮观的钟楼”。这是我第一次阅读国外名著时有这种可碰触的真实感,这些拗口的地名、建筑不再是陌生无法想象的虚幻,它曾经就在我的眼前,我眺望过、感叹过这座宏伟的尖塔。突然发觉读万卷书和行万里路必须要结合起来,你才能真正体会到书中所描绘的景象,才能体会到路途景物的灵魂。这样的你才不再是一个“两耳不闻窗外事”的书呆子,才不再是一个只懂拍照到此一游的普通游客。目前我虽然行了一点路,却远远没能读够百本书,更别说万卷。希望自己能够利用这段难得的时光,静心读点杂书,不为博学多才,只为做个能看懂门路的游人吧。