Nuclear Magnetic Resonance

Nuclear magnetic resonance (NMR) spectroscopy is among the most important analytical techniques in the chemical sciences -- with applications ranging from chemical structure elucidation to precision measurement of dynamics in proteins and nucleic acids -- and the imaging modality (MRI) is an invaluable diagnostic tool for non-invasive medical imaging. My research is focused on the development of novel NMR methods in order to redefine what is possible with NMR, with the ultimate goal of revolutionizing chemical and material analysis.

My proposed research program will achieve
(1) NMR in zero to ultralow magnetic fields, enabling high-resolution NMR spectra of heterogeneous and conductive samples;
(2) nuclear spin hyperpolarization to extend the applicability of NMR and MRI; and
(3) nanoscale NMR using quantum sensors, to bring the exquisite specificity of NMR to the single-molecule limit.

Zero- to Ultralow-Field Nuclear Magnetic Resonance and Imaging

Conventional (high-field) NMR is well established as a powerful, noninvasive spectroscopic tool and is regularly employed for molecular structural elucidation and chemical reaction monitoring. There are, however, fundamental limitations that prevent acquisition of high-resolution spectra from heterogeneous samples or those encased in metal.

I have spent the past decade developing ZULF NMR into a technique for precision measurement and chemical analysis, and I am now planning to apply these methods to the study of practically important systems like electrochemical devices (for example, Li-ion batteries). The ability to measure NMR with or without an applied magnetic field allows one to cut out complexities related to magnetic susceptibility and obtain detailed chemical information from measurements of spin-spin couplings and relaxation. This opens the door to high-precision spatially resolved operando spectroscopy in important, but notoriously challenging systems.

Nuclear Spin Hyperpolarization for the Study of Catalysis and Metabolism

The relatively weak coupling of nuclear spins to the environment is both a blessing and a curse for NMR: on the one hand, it enables long spin coherence times and therefore high resolution and chemical specificity -- on the other hand, it means that signal-to-noise ratios are typically much smaller than for other analytical techniques. Much of this issue is related to the limited signal available from equilibrium nuclear spin polarization under practical conditions, typically 10-6 to 10-5. To solve this problem, hyperpolarization techniques, which generate non-equilibrium spin states in order to achieve dramatic signal enhancements, have been a major focus of magnetic-resonance research.

One important application area is fundamental catalysis research, where NMR is one of the key analytical techniques, but few studies have been performed involving actual heterogeneous catalytic processes, largely due to limited sensitivity. I plan to apply hyperpolarization methods, particularly those related to parahydrogen-induced polarization (PHIP), in order to extend the reach of NMR in the study of catalysis. Hydrogenation reactions are particularly important for both the petrochemical and fine chemical industries, and the fact that signal enhancement relies on pairwise hydrogenation provides insight into reaction mechanisms, aiding in the optimization of heterogeneous catalysts, for which single-site hydrogenation with better-defined and structured active centers is a general goal.

Another exciting application is hyperpolarized metabolic MRI, which has been shown to be particularly useful for profiling of potentially cancerous tissues. PHIP-based methods are emerging as portable and cost-effective techniques for the production of hyperpolarized metabolites, which, together with portable MRI equipment, can bring this new technology to the bedside and to patients in developing countries. My research will focus on the development of new hyperpolarized biomarkers, new protocols for low-field MRI, and exciting applications in neuroscience and materials chemistry.

Nanoscale Zero- to Ultralow-Field NMR with Solid-State Quantum Sensors

Even in samples with no net time-averaged polarization, NMR signals have been shown to arise through statistical fluctuations of the nuclear magnetization, sometimes referred to as 'spin noise'. For microscopic samples such as those that can be measured using nitrogen vacancy (NV) centers in diamond, the spin noise can actually be larger than the polarization, even at fields of several Tesla. In the extreme case of a single spin, there is no difference in signal strength for polarized and unpolarized samples, such that the sample can be thought of as being fully polarized. As spin noise is independent of magnetic-field strength, statistical polarization can be detected at arbitrary fields, and NV magnetometry has been shown to be feasible even at zero magnetic field.

My intent is to achieve spin-noise-detected NMR with NV centers at zero and ultralow magnetic fields in order to establish a general and versatile platform for elucidating the structure and dynamics of molecules and materials at the nanoscale. This project involves work in nanoscale NMR spectroscopy, high-spectral-resolution magnetic sensing with NV centers, and nanofluidic engineering. Applications to on-chip metabolomics, confined molecules, thin films, and interfaces are envisioned.

Physics Projects

Ultralight Bosonic Dark Matter

The nature of dark matter, the invisible substance that makes up over 80% of the matter in the universe, is one of the most intriguing mysteries of modern physics. Elucidating the nature of dark matter will profoundly impact our understanding of cosmology, astrophysics, and particle physics, providing insights into the evolution of the Universe and potentially uncovering new physical laws and fundamental forces beyond the Standard Model.

To date, experimental efforts to directly detect dark matter have largely focused on Weakly Interacting Massive Particles (WIMPs). The absence of evidence for WIMPs has reinvigorated efforts to search for ultralight bosonic fields, another class of theoretically well-motivated dark matter candidates, composed of bosons with masses smaller than a few eV. A wide variety of theories predict new spin-0 bosons such as axions and axion-like particles (ALPs) as well as spin-1 bosons such as dark photons. While most experiments searching for ultralight dark matter seek to detect photons produced by the conversion of dark matter particles in strong electromagnetic fields, a new method to search for dark-matter bosonic fields was recently proposed: dark- matter-driven spin-precession, detected via nuclear magnetic resonance (NMR) techniques.

Cartoon-level schematic of the CASPEr concept.

CASPEr, the Cosmic Axion Spin Precession Experiment

The Cosmic Axion Spin Precession Experiment (CASPEr) is a multi-faceted research program using NMR techniques to search for dark-matter-driven spin-precession. The essence of CASPEr is straightforward: under appropriate experimental conditions, axions/ALPs induce spin precession when interacting with nuclear spins. One can treat the ALP-nuclear spin coupling as a pseudo-magnetic field, BALP (t), with amplitude proportional to the coupling strength and oscillation frequency determined by the boson mass.

As shown schematically to the left, it is thus possible to search for ultralight bosonic dark matter using what is essentially a field-swept CW-NMR experiment where the RF driving field B1 is replaced with BALP. When the nuclear Larmor frequency is equal to the dark matter frequency, BALP drives nuclear spin precession into the transverse plane, such that the oscillating magnetization can be measured using an appropriate sensor.

This signal transduction scheme is summarized below, where it should be noted that the range of masses available to the search is determined by the range of achievable B0 values, as well as the nuclear gyromagnetic ratio. Furthermore, the sensitivity of the search to small coupling parameters is limited by the magnetic sensitivity of the detector, as well as the degree of polarization of the nuclear spins. The latter is significant, as the thermal equilibrium spin polarization is typically on the order of 10−5 at room temperature in a 1 T magnet. Production of enhanced, non-equilibrium spin polarization – as discussed above – is thus a critical experimental task. Phase I of CASPEr- Wind is implementing liquid 129Xe polarized via spin-exchange optical pumping.

Next-Generation Nuclear Spin Haloscopes

Under the NuSHIELD (Nuclear Spin Haloscopes for Interactions with Extremely Light Dark Matter) program, I will be developing new, targeted experiments to focus on high-impact boson mass ranges that are not as effectively investigated by the existing CASPEr apparatus. As construction of the Phase-I experiments continues in Mainz for CASPEr-Wind and in Boston for CASPEr-Electric, NuSHIELD’s efforts to investigate new detection and spin-polarization methods in order to extend the reach of CASPEr into unexplored regions of parameter space can also be thought of as CASPEr’s research-and-development branch.

In an early proof-of-concept experiment, I recently led a team to implement an extension of CASPEr-Wind based on ZULF NMR, searching for low-frequency sidebands resulting from dark-matter-induced modulation of nuclear-spin energy levels – we are in the final stages of analysis and will soon publish our results, which provide new experimental limits on ultralight bosonic dark matter. I intend to further apply ZULF-NMR techniques in the form of a resonant search which will take advantage of planned instrumentation advances and preparation of long-term steady-state nuclear spin-polarized samples.

We will further extend the experimental reach by developing methods to hyperpolarize a range of alternative samples featuring a variety of nuclear spins, including 1H, 13C, 15N, and 31P. These “axion-scattering targets” will be implemented both in ZULF experiments and in high-field narrow-band searches which will serve (1) to focus on theoretically motivated mass ranges – such as those predicted by the SMASH model or other Grand Unified Theories – and (2) to develop the techniques required for future broad-band phases of CASPEr.

We will also contribute to hyperpolarization and sample development for CASPEr-Electric, which will involve dynamic nuclear polarization of 207Pb via transient photo-excited electron spins in perovskite ferroelectrics.

Searches for new Nuclear Spin Couplings

Many further “exotic” spin couplings can be expected to arise from new forces/particles beyond the Standard Model of particle physics [40]. Similar to the axion/axion-like particle case discussed earlier, these can appear as “pseudo-magnetic” fields The high resolution of zero- to ultralow-field NMR, along with the ease of reproducible field reversals, makes it an appealing tool in the search for such couplings.

Molecular Parity Violation and Antisymmetric Spin-Spin Couplings

Parity nonconservation (PNC) has been observed in nuclear decays and in atomic spectroscopy. PNC should also be present in the molecular Hamiltonian, although its effects have not yet been observed. One possible avenue for measurement of molecular parity nonconservation is the measurement of parity-violating components of nuclear spin interactions. We have recently demonstrated a proof-of-principle experiment based on high-field NMR that may have the potential to achieve the required sensitivity in the future.

Another potentially appealing option would be to measure parity-nonconserving components of the rank-1 antisymmetric nuclear spin-spin coupling (a nuclear-spin analog of the Dzyaloshinskii-Moriya interaction), but as this interaction has no first-order contribution to high-field NMR spectra, it has never been measured directly. ZULF-NMR, however, is particularly well-suited to the measurement of interactions that do not commute with the high-field Zeeman Hamiltonian, such as the antisymmetric nuclear spin-spin coupling. This coupling is fundamentally related to chirality, and theoretical work has suggested that measurement of such an interaction in achiral molecules may serve as a promising method for measurement of molecular parity nonconservation. Research in this direction will involve precision measurement of spin couplings in molecules oriented by strong electric fields.