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Off-equilibrium quantum magnetism

English title Off-equilibrium quantum magnetism
Applicant Müller Markus
Number 200558
Funding scheme Project funding
Research institution Condensed Matter Theory Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Condensed Matter Physics
Start/End 01.10.2021 - 30.09.2025
Approved amount 465'091.00
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All Disciplines (2)

Condensed Matter Physics
Theoretical Physics

Keywords (7)

Non-equilibrium quantum systems; Hidden magnetic order; Spin liquids; Driven quantum systems; Magnetism; Magnetic qubits; Electronuclear spin complexes

Lay Summary (German)

Gewöhnlich untersucht die Physik Materieeigenschaften im Gleichgewicht. In diesem Projekt untersuchen wir hingegen verschiedene Möglichkeiten, wie Magnete und verwandte Quantensysteme kontrolliert aus dem Gleichgewicht gebracht werden können, um einerseits neuartige Phänomene zu erzeugen und andererseits Eigenschaften von interessanten Quantenzuständen ans Licht zu bringen, die sich im Gleichgewicht nur schwer detektieren lassen.
Lay summary
Inhalt und Ziele des Forschungsprojekts

Unser Ziel ist es, Nicht-Gleichgewichtszustände von Quantenmagneten zu studieren und zu optimieren, sowie bestehende Analysemethoden um neuartige Nichtgleichgewichtsprotokolle zu erweitern. Konkrete Ziele sind die neuartige Diagnose von exotischen Anregungen in Spinflüssigkeiten, welche keine klassische Ordnung, sondern nur versteckte „Quantenordnung“ aufweisen, die noch nie direkt nachgewiesen werden konnte. Weitere Ziele sind die Induktion magnetischer Ordnung durch gezielte Anregung mit Laserlicht, und damit sehr schnelles magnetisches Schalten, oder das gezielte Kühlen von Kernspins, das für medizinische und quantentechnologische Anwendungen wichtig ist.  Gegenwärtig ist weitgehend unklar, welche magnetischen Komponenten und Protokolle optimal sind, um die erwünschten Phänomene zu beobachten.

Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts

Die Kontrolle magnetischer Ordnung oder einzelner magnetischer Freiheitsgrade hat viele Anwendungen: Als magnetische Schaltelemente für energiesparende Logik, die auf neutralen Spins statt geladenen Elektronen aufbaut; in Form von gekühlten Kernspins für NMR Diagnostik in Physik und Medizin; oder via der Kontrolle gekoppelter elektronischer und Kernspins für Quanteninformationstechnologie und Quantencomputing. Die theoretische Forschung dieses Projekts wird Hand in Hand mit Experimenten insbesondere vor Ort am Paul Scherrer Institut gehen, wo die gewonnen Erkenntnisse umgesetzt und weiterentwickelt werden können.


Non-equilibrium systems, driven quantum systems, magnetism, spin liquids, magnetic qubits, hidden magnetic order, electro-nuclear spin complexes
Direct link to Lay Summary Last update: 31.03.2021

Responsible applicant and co-applicants


Name Institute

Associated projects

Number Title Start Funding scheme
166271 Nonlinear Probes of quantum localized systems 01.03.2017 Project funding


In this project we explore multiple ways to probe, drive and control quantum magnets out of equilibrium. This extends the range of possible probes and phenomena well beyond the narrower realm of equilibrium physics. The project has two parts. The first one analyzes quantum spin liquids and related frustrated systems that avoid long range order down to low temperatures, focussing on new off-equilibrium aspects. The second part studies protocols to drive and control quantum magnets with various aims: to induce hidden magnetic order, to efficiently cool quantum matter, to probe many-body localization in solid state magnets as well as to identify new entangled states of several electron and nuclear spins that might serve as highly coherent quantum memories. Quantum spin liquids are notoriously difficult to diagnose as topological phases with fractionalized excitations, because standard equilibrium stimuli do not couple sufficiently directly to those excitations. Here we introduce a new paradigm of a non-equilibrium probe that paves a way out: We suggest that spin liquids reveal their nature in driven, non-equilibrium steady states: Instead of probing the magnetic response to an external field, we instead break time reversal symmetry by driving a current and thereby probe an entirely different magnetic response. In the vicinity of defects we anticipate the fermionic excitations of gapless spin liquids to result in out-of-equilibrium Friedel oscillations, observable in the local magnetization. This illustrates how patterns in a driven steady state may reveal properties of correlated states that are not visible in equilibrium. Spin liquids are fascinating systems because, as a result of strong frustration, they do not order despite strong correlations. Recent experiments at PSI have shown several instances of strongly correlated, yet not bulk-ordered behavior in layered materials, where the inter-layer coupling is frustrated in different manners: in a Weyl semimetal and in a cuprate superconductor. This project attempts to understand the frustration mechanisms which enable such liquid behavior and suppresses the bulk order. We conjecture the frustrated cuprate superconductor (morally a frustrated XY model) to be in a glassy phase featuring off-equilibrium properties, such as memory and slow relaxation. If confirmed this would be a novelty in the field of superconductors. The second part of the project explores whether rare earth doped hosts can be optically driven to exhibit hidden (non-equilibrium) metastable magnetic phases, in a similar fashion as hidden electronic charge orders were recently induced in electronic systems. In particular we will study the feasibility of fast optical switching into magnetic order out of a paramagnetic ground state by pumping crystal field levels that carry magnetic moments. If successful this opens the door for ultrafast magnetic switches on demand. However, such order requires a low entropy or effective temperature, which requires driving at well-chosen frequencies that may have to be carefully adjusted while pumping. While various schemes of light-driven cooling have been proposed, no comprehensive understanding of the different mechanisms nor of optimal protocols is yet achieved. It is a main goal of this project to fill this gap. Successful completion of this program will provide guidelines how to efficiently cool matter by smart driving, having impact far beyond quantum condensed matter. In particular we hope to improve the cooling efficiency of dynamic nuclear polarization, used to cool and hyperpolarize nuclear spins for NMR applications in medicine, or as means to reduce magnetic noise in solids.