The spread of the Cooper pairs into the ferromagnet in proximity coupled superconductor – ferromagnet (SF) structures is shown to cause a strong inverse electrodynamic phenomenon, namely, the long-range transfer of the magnetic field from the ferromagnet to the superconductor. Contrary to the previously investigated inverse proximity effect resulting from the spin polarization of superconducting surface layer, we found a very generic orbital mechanism of the magnetic moment transfer from a ferromagnet to a superconductor which is unavoidable in S/F hybrids. It is related with the fact that the common superconducting wave function in S and F (near the interface) does not permit to exclude the vector-potential of the magnetization by gauge transformation. From the experimental point of view, this phenomenon reminds the Aharonov-Bohm effect since the current inside the attached superconductor is induced by the ferromagnetic layer which cannot create the magnetic field in the outside in the absence of such superconducting environment. At the same time, the true physical key point is that the wave function penetrating the ferromagnet is responsible for this effect. Let us stress that the characteristic length of the above proposed inverse electrodynamic effect is of the order of the London penetration depth.

Проведено теоретическое сопровождение данных по измерению спиновой восприимчивости при разных температурах и электронных концентрациях двумерной электронной системы на основе кремниевого полевого транзистора в группе В.М. Пудалова (ФИАН).
Сообщение основано на работе V. M. Pudalov, A. Yu. Kuntsevich, M. E. Gershenson, I. S. Burmistrov, and M. Reznikov, Phys. Rev. B 98, 155109 (2018).

Рассмотрен небольшой ферромагнетик во внешнем магнитном поле в присутствие индуцированного спинового тока. В работе выведны квазиклассические уравнения движения для намагниченности, причем эффекты динамической неравновесной функции распределения учтены самосогласованно. Выведено уравнение Ландау-Лифшица-Гильберта, дополненное членом Слончевского для спинового крутящего момента. Найдены устойчивые режимы стационарной прецессии намагниченности, в которых, как оказывается, происходит усиление термоэлектрического тока.
Доклад основан на работе T. Ludwig, I.S. Burmistrov, Y. Gefen, A. Shnirman, "A thermally driven spin-transfer-torque system far from equilibrium: enhancement of the thermoelectric current via pumping current", arxiv:1808.01192

Развита теория оптической спектроскопии неравновесных спиновых и зарядовых флуктуаций в квантовой точке, помещённой в микрорезонатор. Показано, что в режиме сильной связи фотонной моды микрорезонатора с трионным резонансом квантовой точки флуктуации коэффициента прохождения света через микрорезонатор содержат информацию как о спиновой динамике системы, так и о статистике фотонов в микрорезонаторе. В режиме слабой связи возможно формирование "одетых фотонами" состояний, что позволяет определять параметры спиновой спиновой динамики не только для основного, но и для возбуждённого состояния квантовой точки по флуктуациям оптических сигналов Фарадея или Керра. Приложение к системе продольного магнитного поля приводит к возможности детектирования зарядовых флуктуаций в квантовой точке.

This will include e.g. works like
[1] R. Klees, G. Rastelli, J.C. Cuevas, and W. Belzig, "Quantum Geometry of Josephson Matter" (submitted to Science Advances, not on arxiv yet).
[2] Tomohiro Yokoyama, Johannes Reutlinger, Wolfgang Belzig, and Yuli V. Nazarov, "Order, disorder and tunable gaps in the spectrum of Andreev bound states in a multiterminal superconducting device", Phys. Rev. B 95, 045411 (2017) [arXiv:1609.05455].
[3] J. Reutlinger, L. Glazman, Yu. V. Nazarov, and W. Belzig, ""Smile" gap in the density of states of a cavity between superconductors", Phys. Rev. Lett. 112, 067001 (2014) [arXiv:1308.2529].

Presentations: 1.QuantumGeomJJ.pdf, 2.SmileTopo.pdf

Weak, almost non-invasive quantum measurements differ from the standard text book example of strongly invasive, projective measurements, since they leave the measured system basically unchanged. This opens the path to measure e.g. non-commuting observables and at the same time poses several open questions: Which order of operators is measured? Can quantum tests like Bell or Leggett-Garg be reformulated? What time scales are involved in the measurement process? We will address some basic properties of weak measurements leading to surprises like apparent spontaneous time-reversal symmetry breaking or the possibility of engineered detectors to tailor the measured quantum correlations.

Exotic quasiparticles have been observed in complex spin systems exhibiting spin ice rules, skyrmions and so on. Here, we discuss the emergence of novel quasiparticles, mediated by magnetic dipolar interactions that have been hiding in simpler spin systems with uniformly ordered ground states. Amongst other properties, these quasiparticles exhibit a spin ranging from zero to above 1ℏ [1]. Of particular interest is our finding that the eigenmodes in an easy-axis antiferromagnet are spin-zero quasiparticles instead of the widely believed spin-1 magnons [2]. These unusual properties originate from a competition between quantum mechanical squeezing (increasing the spin) and hybridization (decreasing the spin).
We then present a theoretical study of spin transport across a ferrimagnet/non-magnetic conductor interface, when a magnetic eigenmode is driven into a coherent state. In the simple case of ferromagnets with non-integer “effective spin” above 1ℏ, we show that spin-current noise measurement can reveal this fundamental quantum phenomenon [1]. This is in full analogy to the effective charge known e.g. in the fractional quantum Hall regime, which has been experimentally determined via shot noise measurements.
Furthermore, we extend our model to continuously encompasses systems from ferromagnets to antiferromagnets [3] and include novel dissipation terms [4], thereby allowing analytical results for the full range of materials within a unified description. We also allow arbitrary interfaces (disordered and asymmetric). The obtained spin current expression includes intra- as well as cross-sublattice terms. We find that the cross-sublattice terms, disregarded in previous studies, play an important role and result in qualitative changes to our understanding of spin pumping in antiferromagnets. The dc current is found to be sensitive to the asymmetry in interfacial coupling between the two sublattice magnetizations and the mobile electrons, especially for antiferromagnets.
References:
[1] A. Kamra and W. Belzig, Super-Poissonian shot noise of squeezed-magnon mediated spin transport, Phys. Rev. Lett. 116, 146601 (2016).
[2] A. Kamra, U. Agrawal, and W. Belzig, Noninteger-spin magnonic excitations in untextured magnets, Phys. Rev. B 96, 020411(R) (2017).
[3] A. Kamra and W. Belzig, Spin pumping and shot noise in ferrimagnets: bridging ferro- and antiferromagnets, Phys. Rev. Lett. 119, 197201 (2017).
[4] A. Kamra, R. E. Troncoso, W. Belzig, and A. Brataas, Gilbert damping phenomenology for two-sublattice magnets, arXiv:1808.04385 (to appear in Phys. Rev. B).

Способы достижения температур уровня нанокельвинов в атомных газах станут первой темой доклада. Далее будут приведены примеры интересных экспериментов с квантовыми газами. В то же время речь пойдёт о сложностях, которые встречаются в экспериментах и о возможных, ещё не реализованных, путях их преодоления. В заключении подробно будет обсуждаться неопубликованный эксперимент по интерференции цепочки бозе-конденсатов, в котором возникает пространственно периодическая интерференционная картина, несмотря на то, что относительные фазы конденсатов случайны.