Leipzig Spin Resonance Colloquium

LSR Colloquium

Current Program

Spin Resonance with Single Atoms and Molecules on Surfaces

In this talk, I will introduce a new architecture for coherent control of spins on surfaces, by combining electron spin resonance (ESR) and scanning tunneling microscopy (STM) [1]. This technique allows to address single atoms and molecules on surfaces with an unprecedented combination of spatial and energy resolution. Thus, it can be used to sense the magnetic coupling between spin centers on the atomic scale [2], including their dynamics [3,4]. In addition, when scanning the STM tip across the surface it permits to perform magnetic resonance imaging on the atomic scale [5]. The high energy resolution compared to conventional scanning tunneling spectroscopy also grants access to the hyperfine interaction between the electron and nuclear spin of different atomic species [6]. Recently, we could extend this technique also to spin resonance on individual molecules [7]. Lastly, by employing pulsed ESR schemes, a coherent manipulation of the surface spin becomes possible, for instance in Rabi and Hahn echo schemes [8,9]. This opens up a path towards quantum information processing and quantum sensing using atomic building blocks, including atoms and molecules.

[1] S. Baumann, W. Paul et al., Science 350, 6259 (2015).

[2] T. Choi et al. Nature Nanotechnology 12, 420-424 (2017).

[3] F. Natterer et al., Nature 543, 226 (2017).

[4] L. Veldman, L. Farinacci et al., 372, 964-968 (2021).

[5] P. Willke et al., Nature Physics 15, 1005–1010 (2019).

[6] P. Willke et al., Science 362, 336–339 (2018).

[7] X. Zhang et al., Nature Chemistry 14, 59–65 (2022).

[8] K. Yang et al., Science 366, 509-512 (2019).

[9] Y. Wang et al., Science 382, 87-92 (2023)


enlarge the image: Privite photo: P. Willke

THz magnetic resonance

Magnetic resonance spectroscopy has been carried out in the frequency range of 100 to 750 GHz. Resonances in antiferromagnets at zero applied magnetic field were detected using a vector network analyzer (VNA) with frequency extenders. Strong coupling was observed between the electromagnetic modes of a cavity and resonances in antiferromagnets. Coupling between two resonators via cavity modes was obtained even when the samples were separated by several millimeters. This works stems from the development of a switchable gyrotron  and the fabrication of wave guides carrying the 250 GHz gyrotron output to an NMR spectrometer for DNP. This waveguide technology led to the creation of a startup, Swissto12, now making satellites using 3D lithography.


enlarge the image: Private photo: JP Ansermet

Mapping static and dynamic biophysical properties of tissue microstructure from advanced diffusion and functional MRI

In living systems, the tissue micro-architecture consists of myriad cellular and subcellular elements whose density, size/shape distributions, composition, and permeability, endow the tissue with its biological functionality. Dynamic transport mechanisms are further critical for maintaining homeostasis and supporting diverse physiological functions such as action potentials and biochemical signaling. Still, how these biophysical properties change over time and how they couple to activity, remains largely unknown. This is mainly due to the difficulty in mapping these properties in-vivo, longitudinally, and with sufficient specificity. Magnetic Resonance Imaging (MRI), with its capacity for longitudinal studies and wealth of microscopic information leading to multiple contrast mechanisms, provides an outstanding opportunity to decipher these phenomena. In this talk I will discuss our recent advances in diffusion and functional MRI, including novel pulse sequences and biophysical modeling of diffusion processes in the microscopic tissue milieu, which provide, for the first time, the sought-after specificity for density, size, and permeability of particular (sub)cellular elements in tissues. I will show new experiments in rodents proving unique power-laws predicted from biophysical models, revealing axon density and size, as well as cell body density and size, along with validations against ground-truth histology and applications in animal models of disease. Evidence for exchange between the intracellular and extracellular space will also be given, along with a first approach for quantitatively mapping permeability in tissue. I will also introduce correlation tensor MRI (CTI), a new approach for source-separation in diffusional kurtosis, that offers surrogate markers of neurite beading effects, thereby further enhancing specificity, especially in stroke. Finally, if time permits, I will touch upon dynamic modulations of neural tissue microstructure upon neural activity, and provide evidence for the
existence of a neuro-morphological coupling in diffusion-weighted functional MRI signals. Future vistas and potential applications will be discussed.

NMR as an effective probe of `hidden order'

Quantum materials can have complex electronic Hamiltonians with multiple unknown terms leading to different emergent phases of matter, each potentially characterized by an order parameter and its topology. A prime example of such complexity arises in strongly-correlated electron systems with strong spin-orbit coupling (SOC). Most experimental probes are sensitive to only a single aspect of an emergent phase, i.e., a local order parameter that couples to the applied field or quasi-particles used for the measurement. As such, it can be difficult to discern which of the potentially competing interactions drives an observed phase transition, as well as their mutual relationships. Consequently, important (hidden) order parameters and/or interactions can remain elusive. Such hidden subdominant orders or interactions often manifest through the appearance of a so-called missing entropy in thermodynamic measurements. In this talk I will present a novel method employed to measure the full spatial distribution of an order parameter without contamination from other coupled degrees of freedom. In essence, to accomplish this - we utilize nuclear spins as local interferometers to perform minimally invasive, phase sensitive measurements of electronic ground states. This technique is in stark contrast to conventional spectral measurements of matter, which measure the rate of re- laxation caused by quasi-particle excitations.


enlarge the image: Private photo: V. Mitrovic

Collective Long-Lived Zero-Quantum Coherences in Aliphatic Chains

In nuclear magnetic resonance (NMR), long-lived coherences (LLCs) constitute a class of zero-quantum (ZQ) coherences that have lifetimes that can be longer than the relaxation lifetimes T2 of transverse magnetization. So far, such coherences have been observed in systems with two coupled spins with spin quantum numbers I = ½, where a term |S⟩⟨T|+|T⟩⟨S| in the density operator corresponds to a coherent superposition between the singlet |S⟩ and the central triplet |T⟩ state. Here we report on the excitation and detection of collective long-lived coherences in AA’MM’XX’ spin systems in molecules containing a chain of at least three methylene (-CH2-) groups. Several variants of excitation by polychromatic spin-lock induced crossing (poly-SLIC) are introduced that can excite a non-uniform distribution of the amplitudes of terms such as |SST⟩⟨SST|, |STS⟩⟨STS|, and |TSS⟩⟨TSS|. Once the radio frequency (RF) fields are switched off, these are not eigenstates, leading to ZQ precession involving all 6 protons, a process that can be understood as a propagation of spin order along the chain of CH2 groups, before their reconversion into observable magnetization by a second poly-SLIC pulse that can be applied to any one or several of the CH2 groups. In the resulting 2D spectra, the w2 domain shows SQ spectra with the chemical shifts of the CH2 groups irradiated during the reconversion, while the w1 dimension shows ZQ signals in absorption mode with linewidths on the order of 0.1 Hz that are not affected by the inhomogeneity of the static magnetic field, but can be broadened by chemical exchange as occurs in drug screening. The ZQ frequencies are primarily determined by differences between vicinal J-couplings.


enlarge the image: Private Photo: G. Bodenhausen

How solid-state NMR can help understand contrast in brain MRI: T1 relaxation and ihMT

Many tissues are complicated composite materials composed of a mixture of different phases. This mix allows for imaging techniques to produce contrast in many different ways. However, the parameters leading to the contrast displayed in vivo are not always clear, and some are frequently misunderstood. I will talk about our efforts to understand the mechanisms underlying some common and emerging MRI techniques including T1 relaxation and inhomogeneous magnetization transfer with a focus on how magnetization transfer between aqueous and semi-solid tissue components plays a crucial role in the outcomes of these experiments.


enlarge the image: Private photo: C. Michal

NMR of water for probing complex materials

Water is a ubiquitous substance. The fact that its deuterated form is readily available makes this molecule even more interesting for NMR studies. By (partially) replacing normal water with D2O in multi-component mixtures, one can selectively measure the deuteron spectra of water and utilize the remaining quadrupole couplings in anisotropic environments. For example, NMR spectroscopy of D2O has been used for many years to study the phase diagrams of aqueous surfactant solutions. The quadrupole splittings in lyotropic liquid crystals depend not only on the concentration but also on the type of phase. The phase structure can often be assigned on the basis of NMR results alone, and single-phase and multiphase regions can be easily distinguished. More recently, the NMR spectra and diffusion of D2O have also been used to study shear-induced orientations and structural changes of lyotropic liquid crystals. Examples of our rheo-NMR studies will be presented, focusing on the peculiar shear-induced behavior of lamellar phases. Finally, the application of water molecules as probes in porous materials is addressed. The aromatic character of the inner surfaces of carbonaceous materials leads to large up-field shifts of the water resonance by several ppm. Initial results from our current investigations of nitrogen-doped carbonaceous materials will be presented


enlarge the image: Private photo: C. Schmidt

High Pressure Quantum Sensing

Pressure alters the physical, chemical and electronic properties of matter. By compressing a material between two opposing brilliant cut diamonds, the diamond anvil cell enables tabletop experiments to reach pressures more than a million times that of atmospheric pressure. Since its development over half a century ago, it has enabled experiments to directly access pressure as a thermodynamic tuning parameter and has had a dramatic impact on quantum science, chemistry and materials physics. Among these impacts, a tremendous amount of recent attention has focused on the discovery of superconductivity in a class of hydrogen-based materials. When compressed to megabar pressures, these so-called super-hydrides are believed to exhibit the highest known critical temperatures, and have led to a nascent field that is equal parts exciting and controversial. Part of this controversy stems from the nature of the tool itself: especially at high pressures, it is tremendously challenging to extract local information from within a diamond anvil cell. 

In this Colloquium, I will describe a new approach to directly "see" the physics inside the science chamber of a diamond anvil cell at ultra-high pressures. The basic idea is deceptively simple: We directly integrate a thin layer of sensors into the surface of the diamond anvil that is actually applying the pressure. I will demonstrate the ability to perform diffraction-limited imaging of both stress fields and magnetism, with the latter allowing us to image the magnetic field expulsion associated with superconductivity. Applying our techniques to cerium hydride, we observe the dual signatures of superconductivity: diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping both the diamagnetic response and flux trapping, we directly image the geometry of superconducting regions, showing marked inhomogeneities at the micron scale.


enlarge the image: Private photo: N. Yao

Program Archive

LSRC coordinator: Dr. Evgeniya Kirillina, contact email