Quantum technologies promise disruptive advances in computing, communication, and sensing. At the heart of these technologies is the qubit – a controllable two-level quantum system. Our group develops molecular spin qubits, which leverage paramagnetic metal ions inside designer molecules. Chemistry gives these qubits a decisive advantage: their quantum properties can be tuned by synthesis, and identical copies can be made in vast numbers and arranged into ordered arrays. We focus on spin clock transitions that shield molecular qubits from magnetic noise, and we work toward linking such qubits into elementary building blocks of quantum logic.

Projects

Molecular Spin Qubits and Clock Transitions

A qubit is the quantum analog of a classical bit: a two-level system, |0〉 and |1〉, that can also exist in any superposition of the two. Many physical systems are being explored as qubits — superconducting circuits, trapped ions, defects in diamond, dopants in silicon — but each faces steep challenges in scaling and error rates. Molecules offer a distinctive route. A spin-bearing molecule is a nanoscale quantum object whose energy levels can be sculpted by chemical design, and, unlike fabricated devices, molecules can be synthesized as vast numbers of identical copies and organized using the tools of supramolecular and coordination chemistry.

The central obstacle for any qubit is decoherence — the loss of the fragile superposition state through interactions with the environment. For molecular spin qubits, that environment includes molecular vibrations and, crucially, the magnetic noise from surrounding nuclear and electronic spins, collectively called the spin bath. Vibrations can be quieted by working at low temperature, and neighboring electron spins can be diluted away, but nuclear spins are harder to escape, because the hydrogen, nitrogen, and other atoms that make up most ligands carry nuclear spin.

Our strategy sidesteps this problem using a phenomenon called a spin clock transition. When two spin states that would normally cross in energy instead avoid each other, they open a small energy gap. Right at that gap, the transition frequency between the two states becomes insensitive to the magnetic field — its slope with respect to field is essentially zero — so the qubit is, to first order, decoupled from the fluctuating spin bath. The name borrows from atomic clocks, which likewise exploit a transition that is stable against perturbations. Operating a qubit at its clock transition can extend its coherence time by nearly an order of magnitude. The catch is that these gaps are difficult to measure, and only a handful of molecules were known to host them; much of our work aims to expand this chemical space and understand what molecular ingredients produce a useful clock transition.

Clock Transitions in Transition Metal Complexes

One productive family is built around the nickel(II) ion in a trigonal-bipyramidal coordination geometry. Such complexes have a spin S = 1 ground state whose sublevels are split by an effect called zero-field splitting; a slight distortion of the molecule mixes two of these sublevels and opens a clock-transition gap. This makes trigonal-bipyramidal Ni(II) complexes a fertile playground, because the molecular structure can be tuned to control the size of the gap.

We prepared a series of complexes, [Ni(tao)X], in which the identity of a single ligand (X = fluoride, chloride, azide, and others) is varied. As X changes, the molecule distorts by differing amounts, and the clock-transition gap shifts in a systematic way. To pin these gaps down we used several complementary probes: high-frequency EPR spectroscopy, far-infrared magnetospectroscopy (which lets us watch the two levels avoid each other as the field approaches zero), and inelastic neutron scattering. The three methods agree well with one another, giving us confidence both in the measured gaps and in the chemical rules that connect molecular distortion to the clock transition. These results establish nickel(II) complexes as viable, tunable clock-transition qubits.

Clock Transitions in Lanthanide Complexes

Lanthanide ions offer a second, complementary route. In these ions the large orbital angular momentum produces a ladder of states, and when the local symmetry of the ligands is matched to a particular ground-state doublet, the two components of that doublet mix and a clock transition appears. This design rule — matching the rotational symmetry of the coordination environment to the state — was central to the first molecular clock-transition qubit ever reported.

We recently showed that the same idea can be carried into an extended, ordered material. By assembling holmium(III) ions into a metal–organic framework — a crystalline scaffold in which the metal centers are bridged by organic linkers into a regular lattice — we created a solid in which each holmium node is a clock-transition qubit, all held in a rigid, periodic arrangement. High-frequency EPR spectroscopy on single crystals mapped the resonances in three dimensions and revealed a clock transition with an unusually large gap of 54.6 GHz, tied to the distorted square-antiprismatic environment of the holmium ion. Interpreting these rich spectra required a new theoretical approach, developed with our collaborators, that combines density-functional theory with an effective spin-Hamiltonian description; the calculations reproduced the measured gap and clarified the crystal-field states responsible for it. This work demonstrates that clock-transition qubits can be organized into ordered arrays — an important step toward scaling molecular qubits up while preserving the very property that protects them.

Research Skills Acquired

Students in this area gain a broad, interdisciplinary set of skills. They learn to design and synthesize coordination compounds, grow single crystals, and determine crystal structures by X-ray diffraction. They characterize the quantum properties of materials by magnetometry and by advanced magnetic-resonance and scattering techniques, such as high-frequency electron paramagnetic resonance, far-infrared magnetospectroscopy, and inelastic neutron scattering. These advanced measurements are carried out at national and international large-scale user facilities. Students also learn to model electronic structure and spin Hamiltonians and to connect the theory with experiment. This blend of synthetic chemistry, physical measurement, and computation prepares young researchers for careers at the forefront of the emerging quantum industry, in academia, national laboratories, and technology companies.

Metallic magnets can host strikingly complex behaviors when competing magnetic exchange interactions couple to features of electronic structure. We use solid-state chemistry to steer intermetallic compounds toward non-trivial magnetic structures (so-called spin textures) that are of fundamental interest and hold promise for next-generation spintronic devices. By mapping the boundaries between conventional magnetic phases and tracking how crystal structure, chemical bonding, and magnetic order evolve together, we search for the chemical conditions that give rise to these unconventional magnetic states.

Projects

Magnetism, Bonding, and Spin Textures

The magnetism of a metal is intimately tied to its electronic structure. Nearly a century ago, the Stoner criterion established that a metal tends to order ferromagnetically when the density of electronic states at the Fermi level is high enough. This is the foundational principle for itinerant magnets, which dominate today's technologies, from the permanent magnets in electric vehicles to the magnetocaloric materials used for magnetic cooling. More recently, solid-state chemists added a bonding perspective: ferromagnetic order is favored when the interactions between magnetic orbitals at the Fermi level are antibonding, while nonbonding interactions favor antiferromagnetism. The nature of these interactions can be quantified with a tool called crystal orbital Hamilton population (COHP) analysis, which lets us read a material's likely magnetic behavior directly from its chemical bonding.

What makes the study of metallic magnets especially exciting is the sensitivity of their magnetic states to small changes in the electronic structure of materials. We leverage this sensitivity to develop a newer class of behaviors that emerge when magnetic interactions compete. Instead of the simple parallel (ferromagnetic) or antiparallel (antiferromagnetic) arrangements, the spins can settle into swirling, non-collinear patterns, known as skyrmions and merons. These spin textures are topologically protected, meaning they behave as robust objects that can be moved with very little energy. That property makes them attractive as information carriers for low-power, high-density data storage and neuromorphic computing, an alternative to conventional semiconductor electronics. Discovering such states in bulk materials, however, has historically been serendipitous. Our goal is to make the search rational.

Magnetic Phase Boundary Mapping

Our strategy, which we call Magnetic Phase Boundary Mapping, is to interrogate the region of composition space where a material sits on the knife's edge between competing magnetic states. Such boundaries can be found where a transition between different structural symmetry occurs or where the interactions between magnetic atoms shift from nonbonding to antibonding near the Fermi level. By doping electrons or holes into a material, we can walk it up to and across these boundaries, watching for the emergence of complex magnetism. The approach was inspired by decades of work on superconductors and thermoelectrics, where similar phase-boundary mapping led to major discoveries.

A clear example comes from our work on kagome metals, materials in which the magnetic atoms form a lattice of corner-sharing triangles known for hosting frustrated magnetism and unusual flat electronic bands. We studied the series YFe₆Ge₆–YCo₆Ge₆, whose end members are strikingly different: YFe₆Ge₆ is an antiferromagnet, while YCo₆Ge₆ shows no magnetic order at all. Our density-functional theory (DFT) calculations traced the difference to where the Fermi level falls relative to a peak in the electronic density of states. As we replaced iron with cobalt, the antiferromagnetic order was gradually suppressed and, beyond a critical composition, gave way to ferromagnetic-like behavior. Tellingly, this change coincided with a change in the crystal symmetry! Compositions poised right at that boundary are exactly where non-collinear magnetic structures are most likely to appear.

A Skyrmion-Like Spin Texture from Chemistry

The most striking result of the Magnetic Phase Boundary Mapping approach came from exploring the compositional space between MnCoGe and MnCoAs, which are ordinary ferromagnets with centrosymmetric crystal structures. At the boundary between these phases, we discovered an entirely new material, MnCoGe_1/3As_2/3, that crystallizes in a non-centrosymmetric structure. That change is decisive: when a crystal loses its center of symmetry, a special antisymmetric magnetic interaction – the Dzyaloshinskii–Moriya interaction – switches on and twists neighboring spins relative to one another.

Neutron diffraction revealed that this new material undergoes two magnetic transitions. Below 236 K it adopts a non-collinear spin arrangement in which the Mn and Co moments rotate by 120° within the ab plane of the hexagonal lattice; below 165 K, additional features in the diffraction data appear, revealing incommensurate, modulated cycloidal spin texture, which looks like a swirling arrangement of magnetic moments reminiscent of magnetic skyrmions. Importantly, this spin texture was produced not by an external field or an engineered thin film but by a deliberate chemical substitution. Additional diffuse scattering hints that the magnetism may be even richer, possibly described by multiple superimposed modulations. This discovery is a proof of concept that the tools of synthetic chemistry can generate the kind of exotic magnetism usually associated with carefully engineered physical systems.

Research Skills Acquired

This research lies at the juncture of solid-state chemistry and condensed-matter physics. Students learn a wide range of synthetic methods, including conventional high-temperature synthesis, arc-melting, flux crystal growth, and chemical vapor transport. The students also master a range of methods to characterization of materials, such as X-ray diffraction, magnetometry, electrical transport, electron microscopy, as well as advanced neutron and X-ray scattering techniques at national and international research facilities. The students also learn to calculate electronic band structures and correlate them to observed physical properties. This fusion of experimental and theoretical skills gives young researchers a broad, cutting-edge perspective on modern materials science and prepares them for careers in academia, national labs, and industry.

Some molecules can be switched between two distinct states by an external stimulus, such as changes in temperature, pressure, electric or magnetic field, or irradiation with light. The resulting two states exhibit dramatic differences in their magnetic, structural, electrical, and optical properties. Our group studies spin-crossover (SCO) materials, a versatile class of such switchable molecular solids, with the goal to translate this molecular bistability to thin films and 2D materials that could serve as active elements in future molecule-based devices.

Projects

Spin Crossover as a Molecular Phase Change

Spin crossover is one of the most striking examples of bistability in molecular materials. In many iron(II) complexes, the six d-electrons can be arranged in two ways: a low-spin state, in which they are fully paired, and a high-spin state, in which several occupy separate orbitals. The two states have nearly the same energy, so the molecule can be tipped from one to the other by a modest stimulus — cooling or heating, applying pressure, or shining light. Because the switch changes the metal–ligand bond lengths, it also changes the molecule's size, color, magnetism, and electrical response, all at once.

What makes spin crossover a true material phase change, rather than just an isolated molecular event, is cooperativity. When the molecules are strongly coupled in a crystal, the switching of one molecule helps trigger its neighbors, so the transition becomes abrupt and often hysteretic — the material "remembers" whether it was last warmed or cooled. This memory effect, together with the large and reversible change in physical properties, is exactly what makes spin-crossover solids appealing as switches, sensors, and memory elements. A particularly attractive feature is that the switch is accompanied by pronounced changes in optical properties, opening the door to using these materials in light-based, or photonic, technologies.

Sublimable Complexes for Thin Films

To use spin-crossover materials in devices, they must be prepared as high-quality thin films — and this is where a fundamental tension arises. Depositing molecules cleanly from the vapor phase (subliming them) usually requires the intermolecular forces in the solid to be weak, but those same forces provide the cooperativity that makes the spin transition abrupt and hysteretic. Weaken the interactions to gain volatility, and the useful, sharp switching is lost.

We resolved this dilemma with an asymmetric molecular design. By decorating one side of an iron(II) complex with bulky tert-butyl groups while leaving the other side free to form strong contacts, we built a molecule whose crystal is compartmentalized: weak, greasy interactions on one face make it sublimable, while efficient contacts on the other face preserve the cooperative coupling between molecules. The resulting complex sublimes readily to grow crystals and thin films, yet retains an abrupt spin transition with a hysteresis loop — and even shows light-induced switching at low temperature. We further showed that the spin transition survives in films only 20 nm thick, and can be read out electrically, demonstrating that asymmetric design is a general route to sublimable materials with strongly cooperative transitions.

Spin Crossover in Two Dimensions

The same asymmetric design has an unexpected bonus. Because the bulky groups create weakly bound layers held together only by van der Waals forces — the same kind of bonding that holds graphite together — the crystals can be mechanically exfoliated, peeled apart with adhesive tape, down to a single molecular layer. This makes the material a molecular analog of two-dimensional materials such as graphene, but built from discrete, switchable magnetic molecules rather than a covalent sheet.

These exfoliated flakes, as thin as a single ~1.7 nm layer, keep their molecular structure intact and still undergo spin crossover. Remarkably, the switching behavior changes dramatically with thickness: as the flakes are thinned toward the monolayer limit, the hysteresis of the transition widens by more than 100 K, a consequence of confining the cooperative switching to two dimensions and of interactions with the supporting surface. Because these single-crystalline molecular layers can be stacked with other two-dimensional materials into heterostructures, they offer a route to integrating switchable molecular functionality into compact devices — and a clean platform for understanding how a molecular phase change behaves when squeezed into a single layer.

Research Skills Acquired

Students working on this project learn to design and synthesize organic ligands and metal complexes, grow single crystals, and determine crystal structures by X-ray diffraction. They characterize spin-state switching by magnetometry, calorimetry, optical spectroscopy, and variable-temperature diffraction. They also learn to prepare 2D materials by sublimation and mechanical exfoliation, characterizing these ultrathin structures by atomic force microscopy and optical spectroscopy. This set of skills, combining synthetic chemistry, physical measurements, and device fabrication, prepares students and postdocs for careers spanning chemistry, materials science, and the electronics and photonics industries.