Research summary and highlights

The overall objective of our research program is to explore the chemistry and physics of quantum materials. We employ a variety of spectroscopic methods to reveal the properties of these systems, and we use different external stimuli - such as magnetic field, light, pressure, strain, and confinement - to tune and control these properties. The goal is to discover new states of matter or physical phenomena and, at the same time, to uncover properties of technological relevance. Current research directions include (i) unraveling coupling processes in multiferroics, (ii) uncovering functionality from enhanced spin-orbit coupling and ultra-low symmetry in 4- and 5d-containing materials, (iii) revealing pressure-tmperature phase diagrams of chalcogenides, and (iv) controlling the response of nanoscale materials, interfaces, and defect structures.

Real-space infrared spectroscopy of ferroelectric domain walls in multiferroic h-(Lu,Sc)FeO3

We employ synchrotron-based near-field infrared spectroscopy to image the phononic properties of ferroelectric domain walls in hexagonal (h) Lu0.6Sc0.4FeO3, and we compare ourfindings with a detailed symmetry analysis, lattice dynamics calculations, and prior models of domain-wall structure. Rather than metallic and atomically thin as observed in the rare-earth manganites, ferroelectric walls in h-Lu0.6Sc0.4FeO3 are broad and semiconducting, a finding that we attribute to the presence of an A-site substitution-induced intermediate phase that reduces strain and renders the interior of the domain wall nonpolar. Mixed Lu/Sc occupation on the A site also provides compositional heterogeneity over micron-sized length scales, and we leverage the fact that Lu and Sc cluster in different ratios to demonstrate that the spectral characteristics at the wall are robust even in different compositional regimes. This work opens the door to broadband imaging of physical and chemical heterogeneity in ferroics and represents an important step toward revealing the rich properties of these flexibledefect states.
Insulator-metal transition in CrSiTe3 triggered by structural distortion under pressure

van der Waals solids are well known to host remarkable phase diagrams with competing phases, unusual energy transfer processes, and elusive states of matter. Among this class of materials, chalcogenides have emerged as the most ?exible and relevant platforms for unraveling charge-structure-function relationships. In order to explore the properties of complex chalcogenides under external stimuli, we measured the far infrared spectroscopic response of CrSiTe3 under extreme pressure-temperature conditions. Analysis of the 368 cm-1 Si-Te stretching mode and the manner in which it is screened by the closure of the indirect gap reveals that the insulator-metal transition takes place immediately after the structural phase transition - once the mixed phase aspect of the lattice distortion is resolved. At the same time, the two-phase region associated with the structural transition widens with decreasing temperature, and the slope of the insulator-metal transition under pressure is consistent with increasing entropy. These trends completely revise the character of the temperature-pressure phase diagram as well as the relationship between the structural and insulator-metal transitions, leading to a critical nexus of activity that may hide a quantum critical point and allow superconductivity to emerge.
Symmetry progression and possible polar metallicity in NiPS3 under pressure

van der Waals solids are ideal platforms for the discovery of new states of matter and emergent properties under external stimuli. Under pressure, complex chalcogenides like MPS3 (M is Mn, Fe, Ni, V) host sliding and structural transitions, insulator-to-metal transitions, the possibility of an orbitally-selective Mott state, piezochromism, and superconductivity. In this work, we bring together diamond anvil cell techniques, infrared and Raman scattering spectroscopies, and x-ray diffraction with a detailed symmetry analysis and first-principles calculations to uncover a series of high-pressure phases in NiPS3. Remarkably, we find five different states of matter between ambient conditions and 39 GPa - quite different than in the other MPS3 materials. Even more strikingly, infrared spectroscopy and X-ray diffraction combined with a symmetry analysis reveal both metallicity and loss of the inversion center above 23 GPa suggesting that NiPS3 may be a polar metal with a P3m1 space group under these conditions and P1 symmetry under maximum compression. In addition to identifying a candidate polar metal ripe for further inquiry, we suggest that pressure may tune other complex chalcogenides into this elusive state.
Nonreciprocal directional dichroism at telecom wavelengths

Magnetoelectrics with ultra-low symmetry and spin-orbit coupling are well known to display a number of remarkable properties including nonreciprocal directional dichroism. As a polar and chiral magnet, Ni3TeO6 is predicted to host this effect in three fundamentally different configurations, although only two have been experimentally verified. Inspired by the opportunity to unravel the structure-property relations of such a unique light-matter interaction, we combined magneto-optical spectroscopy and first-principles calculations to reveal nonreciprocity in the toroidal geometry and compared our findings with the chiral configurations. We find that formation of Ni toroidal moments is responsible for the largest effects near 1.1 eV - a tendency that is captured by our microscopic model and computational implementation. At the same time, we demonstrate deterministic control of nonreciprocal directional dichroism in Ni3TeO6 across the entire telecom wavelength range. This discovery will accelerate the development of photonics applications that take advantage of unusual symmetry characteristics.
Vibrational fingerprints of ferroelectric HfO2

Hafnia (HfO2) is a promising material for emerging chip applications due to its high-kappa dielectric behavior, suitability for negative capacitance heterostructures, scalable ferroelectricity, and silicon compatibility. The lattice dynamics along with phononic properties such as thermal conductivity, contraction, and heat capacity are under-explored, primarily due to the absence of high quality single crystals. Herein, we report the vibrational properties of a series of HfO2 crystals stabilized with Yttrium (chemical formula HfO2:xY, where x is 20, 12, 11, 8, and 0%) and compare our findings with a symmetry analysis and lattice dynamics calculations. We untangle the effects of Y by testing our calculations against the measured Raman and infrared spectra of the cubic, antipolar orthorhombic, and monoclinic phases and then proceed to reveal the signature modes of polar orthorhombic hafnia. This work provides a spectroscopic fingerprint for several different phases of HfO2 and paves the way for an analysis of mode contributions to high-kappa dielectric and ferroelectric properties for chip technologies.
Exploring few and single layer CrPS4 with near-field infrared spectroscopy

We combine synchrotron-based near-field infrared spectroscopy and first principles lattice dynamics calculations to explore the vibrational response of CrPS4 in bulk, few-, and single-layer form. Analysis of the mode pattern reveals a C2 polar + chiral space group, no symmetry crossover as a function of layer number, and a series of non-monotonic frequency shifts in which modes with significant intralayer character harden on approach to the ultra-thin limit whereas those containing interlayer motion or more complicated displacement patterns soften and show inflection points or steps. This is different from MnPS3 where phonons shift as 1/size2 and are sensitive to the three-fold rotation about the metal center that drives the symmetry crossover. We discuss these differences as well as implications for properties such as electric polarization in terms of presence or absence of the P-P dimer and other aspects of local structure, sheet density, and size of the van der Waals gap.
Chemical bonding and Born charge in 1T-HfS2

We combine infrared absorption and Raman scattering spectroscopies to explore the properties of the heavy transition metal dichalcogenide 1T-HfS2. We employ the LO-TO splitting of the Eu vibrational mode along with a reevaluation of mode mass, unit cell volume, and dielectric constant to reveal the Born effective charge. We find ZB* is 5.3e-, in excellent agreement with complementary first-principles calculations. In addition to resolving the controversy over the nature of chemical bonding in this system, we decompose Born charge into polarizability and local charge. We find that polarizability is 5.07 A3 and Z* is 5.2e-, respectively. Polar displacement-induced charge transfer from sulfur p to hafnium d is responsible for the enhanced Born charge compared to the nominal 4+ expected for hafnium. 1T-HfS2 is thus an ionic crystal with strong and dynamic covalent effects. Taken together, our work places the vibrational properties of 1T-HfS2 on a firm foundation and opens the door to understanding the properties of tubes and sheets.
Excitations of intercalated metal monolayers in transition metal dichalcogenides

We combined Raman scattering spectroscopy and lattice dynamics calculations to reveal the fundamental excitations of the intercalated metal monolayers in the FexTaS2 family of materials. Both in- and out-of-plane modes are identified, each of which has trends that depend upon the metal-metal distance, the size of the van der Waals gap, and the metal-to-chalcogenide slab mass ratio. We test these trends against the response of similar systems including Cr-intercalated NbS2 and RbFe(SO4)2 and demonstrate that the metal monolayer excitations are both coherent and tunable. We discuss the consequences of intercalated metal monolayer excitations for material properties and developing applications.
Site-specific spectroscopic measurement of spin and charge in (LuFeO3)m/(LuFe2O4)1 multiferroic superlattices

Interface materials offer a means to achieve electrical control of ferrimagnetism at room temperature as was recently demonstrated in (LuFeO3)m/(LuFe2O4)1 superlattices. A challenge to understanding the inner workings of these complex magnetoelectric multiferroics is the multitude of distinct Fe centres and their associated environments. This is because macroscopic techniques characterize average responses rather than the role of individual iron centres. Here, we combine optical absorption, magnetic circular dichroism and first-principles calculations to uncover the origin of high-temperature magnetism in these superlattices and the charge-ordering pattern in the m
Piezochromism in the magnetic chalcogenide MnPS3

van der Waals materials are exceptionally responsive to external stimuli. Pressure-induced layer sliding, metallicity, and superconductivity are fascinating examples. Inspired by opportunities in this area, we combined high pressure optical spectroscopies and first-principles calculations to reveal piezochromism in MnPS3. Dramatic color changes (green to yellow to red to black) take place as the charge gap shifts across the visible regime and into the near infrared, moving systematically toward closure at a rate of approximately -50 meV/GPa. This effect is quenched by the appearance of the insulator-metal transition. In addition to uncovering an intriguing and tunable functionality that is likely to appear in other complex chalcogenides, the discovery that piezochromism can be deterministically controlled at room temperature accelerates the development of technologies that take advantage of stress-activated modification of electronic structure.
Nonreciprocal directional dichroism of a chiral magnet in the visible range

Nonreciprocal directional dichroism is an unusual light-matter interaction that gives rise to diode-like behavior in low symmetry materials. The chiral varieties are particularly scarce due to the requirements for strong spin-orbit coupling, broken time reversal symmetry, and a chiral axis. Here, we bring together magneto-optical spectroscopy and first principles calculations to reveal high energy, broad band nonreciprocal directional dichroism in Ni3TeO6 with special focus on behavior in the metamagnetic phase above 52 T. In addition to demonstrating this effect in the magnetochiral configuration, we explore the transverse magnetochiral orientation in which applied field and light propagation are orthogonal to the chiral axis and by so doing, uncover an additional configuration with a unique nonreciprocal response in the visible part of the spectrum. In a significant conceptual advance, we use first-principles methods to analyze how the Ni2+ d-to-d on-site excitations develop magnetoelectric character and present a microscopic model that unlocks the door to theory-driven discovery of chiral magnets with nonreciprocal properties.
Infrared nano-spectroscopy of ferroelastic domain walls in hybrid improper ferroelectric Ca3Ti2O7

Ferroic materials are well known to exhibit heterogeneity in the form of domain walls. Understanding the properties of these boundaries is crucial for controlling functionality with external stimuli and for realizing their potential for ultra-low power memory and logic devices as well as novel computing architectures. In this work, we employ synchrotron-based near field infrared nano-spectroscopy to reveal the vibrational properties of ferroelastic (90 degree ferroelectric) domain walls in the hybrid improper ferroelectric Ca3Ti2O7. By locally mapping the Ti-O stretching and Ti-O-Ti bending modes, we reveal how structural order parameters rotate across a wall. Thus, we link observed near-field amplitude changes to underlying structural modulations and test ferroelectric switching models against real space measurements of local structure. This initiative opens the door to broadband infrared nanoimaging of heterogeneity in ferroics.
Spin-lattice and electron-phonon coupling in 3d/5d hybrid Sr3NiIrO6

While 3d-containing materials display strong electron correlations, narrow band widths, and robust magnetism, 5d systems are recognized for strong spin-orbit coupling, increased hybridization, and more diffuse orbitals. Combining these properties leads to novel behavior. Sr3NiIrO6, for example, displays complex magnetism and ultra-high coercive fields - up to an incredible 55 T. We combine infrared and optical spectroscopies with high-field magnetization and first principles calculations to explore the fundamental excitations of the lattice and related coupling processes including spin-lattice and electron-phonon mechanisms. Magneto-infrared spectroscopy reveals spin-lattice coupling of three phonons that modulate the Ir environment to reduce the energy required to modify the spin arrangement. While these modes primarily affect exchange within the chains, analysis also uncovers important inter-chain motion. This provides a mechanism by which inter-chain interactions can occur in the developing model for ultra-high coercivity. At the same time, analysis of the on-site Ir4+ excitations reveals vibronic coupling and extremely large crystal field parameters that lead to a t2g-derived low-spin state for Ir. These findings highlight the spin-charge-lattice entanglement in Sr3NiIrO6 and suggest that similar interactions may take place in other 3d/5d hybrids.
Magnetic field-temperature phase diagram of multiferroic (NH4)2FeCl5.H2O

Owning to their overall low energy scales, flexible molecular architectures, and ease of chemical substitution, molecule-based multiferroics are extraordinarily responsive to external stimuli and exhibit remarkably rich phase diagrams. Even so, the stability and microscopic properties of various magnetic states in close proximity to quantum critical points are highly under-explored in these materials. Inspired by these opportunities, we combined pulsed-field magnetization, first-principles calculations, and numerical simulations to reveal the magnetic field - temperature (B - T) phase diagram of multiferroic (NH4)2FeCl5.H2O. In this system, a network of intermolecular hydrogen and halogen bonds creates a competing set of exchange interactions that generates additional structure in the phase diagram - both in the vicinity of the spin flop and near the 30 T transition to the fully saturated state. Consequently, the phase diagrams of (NH4)2FeCl5.H2O and its deuterated analog are much more complex than those of other molecule-based multiferroics. The entire series of coupled electric and magnetic transitions can be accessed with a powered magnet, opening the door to exploration and control of properties in this and related materials.
Charge and bonding in CuGeO3 nanoparticles

We combine infrared and Raman spectroscopies to investigate finite length scale effects in CuGeO3 nanorods. The infrared-active phonons display remarkably strong size dependence whereas the Raman-active features are, by comparison, nearly rigid. A splitting analysis of the Davydov pairs reveals complex changes in chemical bonding with rod length and temperature. Near the spin-Peierls transition, stronger intralayer bonding in the smallest rods indicates a more rigid lattice which helps to suppress the spin-Peierls transition. Taken together, these findings advance the understanding of size effects and collective phase transitions in low-dimensional oxides.
Competing magnetostructural phases in a semiclassical system

The interplay between charge, structure, and magnetism gives rise to rich phase diagrams in complex materials with exotic properties emerging when phases compete. Molecule-based materials are particularly advantageous in this regard due to their low energy scales, flexible lattices, and chemical tunability. Here, we bring together high pressure Raman scattering, modeling, and first principles calculations to reveal the pressure-temperature-magnetic field phase diagram of Mn[N(CN)2]2. We uncover how hidden soft modes involving octahedral rotations drive two pressure-induced transitions triggering the low to high magnetic anisotropy crossover and a unique reorientation of exchange planes. These magnetostructural transitions and their mechanisms highlight the importance of spin-lattice interactions in establishing phases with novel magnetic properties in Mn(II)-containing systems.
Magnetoelectric coupling through the spin flop transition in Ni3TeO6

We combined high field optical spectroscopy and first principles calculations to analyze the electronic structure of Ni3TeO6 across the 53 K and 9 T magnetic transitions, both of which are accompanied by large changes in electric polarization. The color properties are sensitive to magnetic order due to field-induced changes in the crystal field environment, with those around Ni1 and Ni2 most affected. These findings advance the understanding of magnetoelectric coupling in materials in which magnetic 3d centers coexist with nonmagnetic heavy chalcogenide cations.
High pressure vibrational properties of WS2 nanotubes

We bring together synchrotron-based infrared and Raman spectroscopies, diamond anvil cell techniques, and an analysis of frequency shifts and lattice dynamics to unveil the vibrational properties of multiwall WS2 nanotubes under compression. While most of the vibrational modes display similar hardening trends, the Raman-active A1g breathing mode is almost twice as responsive, suggesting that the nanotube breakdown pathway under strain proceeds through this displacement. At the same time, the previously unexplored high pressure infrared response provides unexpected insight into the electronic properties of the multiwall WS2 tubes. The development of the localized absorption is fit to a percolation model, indicating that the nanotubes display a modest macroscopic conductivity due to hopping from tube to tube.
Pressure-induced magnetic crossover driven by hydrogen bonding in CuF2(H2O)2(3-chloropyridine)

Hydrogen bonding plays a foundational role in the life, earth, and chemical sciences, with its richness and strength depending on the situation. In molecular materials, these interactions determine assembly mechanisms, control superconductivity, and even permit magnetic exchange. In spite of its long-standing importance, exquisite control of hydrogen bonding in molecule-based magnets has only been realized in limited form and remains as one of the major challenges. Here, we report the discovery that pressure can tune the dimensionality of hydrogen bonding networks in CuF2(H2O)2(3-chloropyridine) to induce magnetic switching. Specifically, we reveal how the development of O-H   Cl exchange pathways under compression combined with an enhanced ab-plane hydrogen bonding network yields a three dimensional superexchange web between copper centers that triggers a reversible magnetic crossover. Similar pressure and strain-driven crossover mechanisms involving coordinated motion of hydrogen bond networks may play out in other quantum magnets.
Electron-phonon and magnetoelastic interactions in ferromagnetic Co[N(CN)2]2

We combined Raman and infrared vibrational spectroscopies with complementary lattice dynamics calculations and magnetization measurements to reveal the dynamic aspects of charge-lattice-spin coupling in Co[N(CN)2] 2. Our work uncovers electron-phonon coupling as a magnetic field-driven avoided crossing of the low-lying Co2+ electronic excitation with two ligand phonons and a magnetoelastic effect that signals a flexible local CoN6 environment. Their simultaneous presence indicates the ease with which energy is transferred over multiple length and time scales in this system
Quantum critical transition amplifies magnetoelastic coupling in Mn[N(CN)2]2

We report the discovery of a magnetic quantum critical transition in Mn[N(CN)2]2 that drives the system from a canted antiferromagnetic state to the fully polarized state with amplified magnetoelastic coupling as an intrinsic part of the process. The local lattice distortions, revealed through systematic phonon frequency shifts, suggest a combined MnN6 octahedra distortion + counterrotation mechanism that reduces antiferromagnetic interactions and acts to accommodate the field-induced state. These findings deepen our understanding of magnetoelastic coupling near a magnetic quantum critical point and away from the static limit.
Observation of a Burstein-Moss shift in Rhenium-doped MoS2 Nanoparticles

We investigated the optical properties of rhenium-doped MoS2 nanoparticles and compared our findings with the pristine and bulk analogues. Our measurements reveal that confinement softens the exciton positions and reduces spinorbit coupling, whereas doping has the opposite effect. We model the carrier-induced exciton blue shift in terms of the BursteinMoss effect. These findings are important for understanding doping and finite length scale effects in low-dimensional nanoscale materials.
Spectroscopic signature of the superparamagnetic transition and surface spin disorder in CoFe2O4 nanoparticles

We measured the infrared vibrational properties of CoFe2O4 nanoparticles and compared our results to trends in the coercivity over the same size range and to the response of the bulk material. Remarkably, the spectroscopic response is sensitive to the size-induced crossover to the superparamagnetic state, which occurs between 7 and 10 nm. A spin-phonon coupling analysis supports the core-shell model. Moreover, it provides an estimate of the magnetically disordered shell thickness, which increases from 0.4 nm in the 14 nm particles to 0.8 nm in the 5 nm particles, demonstrating that the associated local lattice distortions take place on the length scale of the unit cell. These findings are important for understanding finite length scale effects in this and other magnetic oxides where magnetoelastic interactions are important.