Seminar experimentelle Physik der kondensierten Materie

Recent advances in optical studies of condensed matter have led to the emergence of a variety of phenomena that have conventionally been studied in quantum optics. These studies have not only deepened our understanding of light-matter interactions but also introduced aspects of many-body effects inherent in condensed matter. This talk will describe our recent studies of Dicke cooperativity, i.e., many-body enhancement of light-matter interaction, a concept in quantum optics, in condensed matter. This enhancement has led to the realization of the ultrastrong coupling (USC) regime, where new phenomena emerge through the breakdown of the rotating wave approximation (RWA). We will first describe our observation of USC in a 2D electron gas in a high-Q terahertz cavity in a magnetic field, and definitive evidence for the vacuum Bloch-Siegert shift, a signature of the breakdown of the RWA. Further, we have shown that cooperative USC also occurs in magnetic solids in the form of matter-matter interaction, i.e., spin-magnon and magnon-magnon interactions in rare earth orthoferrites. Particularly, the exchange interaction of N paramagnetic Er3+ spins with an Fe3+ magnon field in ErFeO3 has exhibited a vacuum Rabi splitting whose magnitude is proportional to N1/2 [6]. In the lowest temperature range, these cooperative interactions lead to a magnonic superradiant phase transition. These results provide a route for understanding, controlling, and predict novel phases of condensed matter.

Recent advances in optical studies of condensed matter have led to the emergence of a variety of phenomena that have conventionally been studied in quantum optics. These studies have not only deepened our understanding of light-matter interactions but also introduced aspects of many-body effects inherent in condensed matter. This talk will describe our recent studies of Dicke cooperativity, i.e., many-body enhancement of light-matter interaction, a concept in quantum optics, in condensed matter. This enhancement has led to the realization of the ultrastrong coupling (USC) regime, where new phenomena emerge through the breakdown of the rotating wave approximation (RWA). We will first describe our observation of USC in a 2D electron gas in a high-Q terahertz cavity in a magnetic field, and definitive evidence for the vacuum Bloch-Siegert shift, a signature of the breakdown of the RWA. Further, we have shown that cooperative USC also occurs in magnetic solids in the form of matter-matter interaction, i.e., spin-magnon and magnon-magnon interactions in rare earth orthoferrites. Particularly, the exchange interaction of N paramagnetic Er3+ spins with an Fe3+ magnon field in ErFeO3 has exhibited a vacuum Rabi splitting whose magnitude is proportional to N1/2 [6]. In the lowest temperature range, these cooperative interactions lead to a magnonic superradiant phase transition. These results provide a route for understanding, controlling, and predict novel phases of condensed matter.

Synthetic antiferromagnets are magnetic multilayers consisting of two or more ferromagnetic layers that are coupled antiferromagnetically. They play an important role in spintronic devices, e.g., as field sensors, and as synthetic materials for fundamental explorations. In this talk, I will highlight the use of synthetic antiferromagnets for quantum information science with spin waves, i.e., for quantum magnonics. Examples that are discussed are unidirectionally-coupled magnetic layers that give rise to magnon quantum amplification, and new ways to entangle magnons between two ferromagnetic layers. Both these examples rely on the possibility to engineer both the interactions between the layers, and the interactions of the magnetic layers with the environment. This tunability highlights the potential of synthetic antiferromagnets for quantum magnonics.

In a world where a single company like Google consumed more energy than a country with population of about 20 million [1] in 2019 and this is growing exponentially, it is essential to find energy efficient approaches to make our computing needs sustainable. One potential solution is the use of nanoscale magnetic computing devices. Towards this end, energy efficient approaches based on electrical field control of nanoscale magnetism are pursued in our group: (i) strain mediated switching of the magnetization of nanomagnets [2]; (ii) creation and annihilation of magnetic skyrmions using direct voltage control of magnetic anisotropy (VCMA) [3]; and more recently magnetoionic control [4]. Such nanoscale magnetic devices have application to non-volatile memory [3], hardware AI [4, 5, 6] and quantum control of spins [7,8,9]. We will discuss skyrmion mediated voltage control of nanoscale magnetization that has potential for extremely energy efficient non-volatile memory [3] and are robust to switching errors in the presence of thermal noise, material and device inhomogeneities, while scaling to lateral dimensions of 20 nm and below [3]. Furthermore, energy efficient AI hardware can be realized with nanomagnetic devices. Multi-state nanoscale domain wall racetracks can be used as highly quantized synapses in deep neural networks [5] and convolutional neural networks, with overall improvement in area, energy, and latency by 13.8, 9.6, and 3.5 times respectively [5] compared to purely CMOS implementations. Additionally, interacting nanomagnets can be used for analog [6] and digital reservoir computing [6] and long-term prediction of temporal data [6]. We will specifically discuss experimental implementation of reservoir computing with magnetoionic devices [4] that do not need conversion of signals to GHz unlike when Spin Torque Nano Oscillators (STNOs) are used. In quantum computing, in addition to energy efficiency, one significant problem is implementing qubits in a scalable manner at temperatures of a few Kelvin. We argue that ensemble spin qubits may offer such a possibility [7]. Furthermore, by driving the magnetization of nanomagnets electrically, highly confined microwaves can be generated at the Larmor precession frequency of proximally located spins [8]. This can implement single-qubit quantum gates with fidelities approaching state-of-the-art in a scalable manner. Further confinement of microwaves using convergent-divergent skyrmion devices can implement even more localized and low footprint quantum control of spins [8]. New experimental and simulation results in these directions will be discussed [9]. References [1] FORBES Editor’s pick, Oct 21, 2020,04:26pm EDT [2] Nano Letters, 16, 1069, 2016; Nano Lett., 16, 5681, 2016; Appl. Phys. Lett. 121, 252401, 2022; https://arxiv.org/abs/2501.00980 [3] Nature Electronics 3, 539, 2020; Scientific Reports,11, 20914, 2021; Scientific Reports, 14, 17199, 2024 [4] https://arxiv.org/abs/2412.06964 [5] Nanotech. 31 145201, 2020, IEEE Access, 10, 84946, 2022; IEEE Trans. on Neural Networks and Learning Sys, 36, 4996, 2024. [6] Appl. Phys. Lett. 121, 102402, 2022; Comm. Phys. 6, 215, 2023; Neuromorph. Comput. Eng. 2 044011; IEEE Access, 11,124725, 2023 [7] https://arxiv.org/abs/2503.12071 [8] Communication Physics 5, 284, 2022; Physical Phys. Rev. Applied 22, 06407, 2024. [9] https://arxiv.org/abs/2407.14018

In this talk, I will review two device concepts based on nonlinear dissipative magnetic dynamics. First, we revisit the problem of spin superfluidity, which has been predicted to facilitate coherent spin transport. We propose to both exhibit and exploit this elusive transport phenomenon via the "spin-superfluid quantum interference device" (spin SQUID) — inspired by its superconducting (rf SQUID) and superfluid helium (SHeQUID) counterparts. In particular, we discuss its potential electric-field sensing functionality based on the microwave response of the simplest pertinent structure: a magnetic ring with a single weak link. In the second part of the talk, we systematically address the pseudo-Hermitian physics of dynamically-coupled magnetic macrospins, with a focus on non-Hermitian mode hybridization and its potential utility as a scalable building block for dynamic Ising machines.