About
Pegeen Ryan is an Associate Professor of the Practice in the College of Communication at Boston University. She joined the college after working on the creative side of the advertising business for 30 years. Her career began as an art director, and she rose to the position of VP Creative Director at various Boston advertising agencies. She spent a significant portion of her career at Digitas, where she served as Creative Director for major brands such as P&G, The Home Depot, GM, and Gillette. Pegeen Ryan has won numerous awards, including Cannes, MITX, Echo, and Hatch. She teaches creative courses for both graduate and undergraduate students, sharing her extensive industry experience and expertise.
Research topics
- Optoelectronics
- Crystallography
- Metallurgy
- Materials science
- Chemistry
- Condensed matter physics
- Physics
- Quantum mechanics
Selected publications
Physical Review X · 2025-04-21 · 4 citations
articleOpen accessCompeting interactions in frustrated magnets can give rise to highly degenerate ground states from which correlated liquidlike states of matter often emerge. The scaling of this degeneracy influences the ultimate ground state, with extensive degeneracies potentially yielding quantum spin liquids, while subextensive or smaller degeneracies yield static orders. A long-standing problem is to understand how ordered states precipitate from this degenerate manifold and what echoes of the degeneracy survive ordering. Here, we use neutron scattering to experimentally demonstrate a new “nodal-line” spin liquid, where spins collectively fluctuate within a subextensive manifold spanning one-dimensional lines in reciprocal space. Realized in the spin-orbit-coupled, face-centered-cubic iridate <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi mathvariant="normal">K</a:mi><a:mn>2</a:mn></a:msub><a:msub><a:mi>IrCl</a:mi><a:mn>6</a:mn></a:msub></a:math>, we show that the subextensive degeneracy is robust, but remains susceptible to fluctuations or longer-range interactions which cooperate to select a magnetic order at low temperatures. Proximity to the nodal-line spin liquid influences the ordered state, enhancing the effects of quantum fluctuations that in turn act to stabilize the sublattice magnetization through the self-consistent opening of a large spin-wave gap. Our results demonstrate how quantum fluctuations can act counterintuitively in frustrated materials: Even in a case where fluctuations are ineffective at selecting an ordered state from a degenerate manifold, at the brink of the nodal spin liquid, they can act to protect the ordered state and dictate its low-energy physics.
Nature Communications · 2025-05-13 · 8 citations
articleOpen accessMechanical strain presents an effective control over symmetry-breaking phase transitions. In quantum paraelectric SrTiO3, strain can induce ferroelectric order via modification of the local Ti potential energy landscape. However, brittle bulk materials can only withstand limited strain range (~0.1%). Taking advantage of nanoscopically-thin freestanding membranes, we demonstrate an in-situ strain-induced reversible ferroelectric transition in freestanding SrTiO3 membranes. We measure the ferroelectric order by detecting the local anisotropy of the Ti 3d orbital signature using X-ray linear dichroism at the Ti-K pre-edge, while the strain is determined by X-ray diffraction. With reduced thickness, the SrTiO3 membranes remain elastic with >1% tensile strain cycles. A robust displacive ferroelectricity appears beyond a temperature-dependent critical strain. Interestingly, we discover a crossover from a classical ferroelectric transition to a quantum regime at low temperatures, which enhances strain-induced ferroelectricity. Our results offer new opportunities to strain engineer functional properties in low dimensional quantum materials and provide new insights into the role of ferroelectric fluctuations in quantum paraelectric SrTiO3. The authors achieve precise strain control of freestanding SrTiO3 membranes, enabling reversible ferroelectric transition. In-situ X-ray absorption spectroscopy unveils classical-to-quantum crossover in ferroelectric phase transition at low temperatures.
Physical review. B./Physical review. B · 2025-08-06
articleSenior authorSymmetry is a central concept in condensed matter physics. Similarly, disorder is pervasive in real materials and can have important effects on a wide range of physical properties. Here, the authors explore how disorder affects a strain-tuned bicritical point associated with the meeting of two non-symmetry-related continuous phase transitions in an orthorhombic material. They find evidence for an emergent pseudo-tetragonal symmetry proximate to the bicritical point, which, despite negligible changes in the structural orthorhombicity, occurs for a smaller value of the critical strain and extends over a wider range of tuning parameters than for the pristine material.
Proceedings of the National Academy of Sciences · 2025-09-08 · 3 citations
articleOpen accessThe iron-based high- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>c</mml:mi> </mml:msub> </mml:math> superconductors (SCs) exhibit rich phase diagrams with intertwined phases, including magnetism, nematicity, and superconductivity. The superconducting <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>c</mml:mi> </mml:msub> </mml:math> in many of these materials is maximized in the regime of strong nematic fluctuations, making the role of nematicity in influencing the superconductivity a topic of intense research. Here, we use the AC elastocaloric effect (ECE) to map out the phase diagram of Ba(Fe 1− x Co x ) 2 As 2 near optimal doping. The ECE signature at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>c</mml:mi> </mml:msub> </mml:math> on the overdoped side, where superconductivity condenses without any nematic order, is quantitatively consistent with other thermodynamic probes that indicate a single-component superconducting state. In contrast, on the slightly underdoped side, where superconductivity condenses within the nematic phase, ECE reveals a second thermodynamic transition proximate to and below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>c</mml:mi> </mml:msub> </mml:math> . We rule out magnetism and reentrant tetragonality as the origin of this transition and find that our observations strongly suggest a phase transition into a multicomponent superconducting state. This implies the existence of a subdominant pairing instability that competes strongly with the dominant <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msup> <mml:mi>s</mml:mi> <mml:mo>±</mml:mo> </mml:msup> </mml:math> instability. Our results highlight the significant role of nematic order in determining the pairing symmetry close to optimal doping in this extensively studied iron-based SC, while also demonstrating the power of ECE in uncovering strain-tuned phase diagrams of quantum materials.
Spectrally sharp magnetic excitations above the critical temperature in a frustrated Weyl semimetal
Nature Communications · 2025-07-17 · 1 citations
articleOpen accessThe rare-earth α-pyrochlore iridates are a prospective class of conducting frustrated magnets where electronic correlations, large spin-orbit coupling, and geometrical frustration interplay, leading to a rich set of magnetic and electronic phases. Despite their intriguing properties, the magnetic order and excitations in this fundamental class of topological quantum materials remain poorly understood due to challenges in growing large single crystals and insufficient microscopic information on their temperature-dependent phases. Here, by combining state-of-the-art thin-film synthesis, resonant elastic and inelastic X-ray scattering, spin wave analysis, and dynamical spin susceptibility calculations, we unequivocally reveal the presence of spectrally sharp, gapped magnetic excitations in Y2Ir2O7 that surprisingly persist well above the Néel transition temperature, signaling the presence of a quasi-universal regime connected to fluctuations on frustrated lattices. This finding implies the existence of a highly unusual cooperative paramagnetic (CP) phase above the ordering temperature and offers an explanation for the puzzling high-temperature magnetic behavior observed across the family of metallic pyrochlore crystals. Understanding such magnetic excitations at technologically relevant temperatures opens up possibilities for novel topological spintronic devices. Cooperative paramagnetism refers to a strongly correlated state without long range magnetic order that occurs in frustrated magnetic systems between the Neel temperature and Curie-Weiss temperature. Here, using resonant elastic magnetic and inelastic x-ray scattering, Terilli et al find a spectrally sharp gapped magnetic excitations that persists above the Neel temperature in Y2Ir2O7, implying a cooperative paramagnetic phase.
Large asymmetric anomalous Nernst effect in the antiferromagnet SrIr0.8Sn0.2O3
Nature Communications · 2025-03-25 · 5 citations
articleOpen accessA large anomalous Nernst effect is essential for thermoelectric energy-harvesting in the transverse geometry without external magnetic field. It’s often connected with anomalous Hall effect, especially when electronic Berry curvature is believed to be the driving force. This approach implicitly assumes the same symmetry for the Nernst and Hall coefficients, which is however not necessarily true. Here we report a large anomalous Nernst effect in antiferromagnetic SrIr0.8Sn0.2O3 that defies the antisymmetric constraint on the anomalous Hall effect imposed by the Onsager reciprocal relation. The observed spontaneous Nernst thermopower quickly reaches the sub-μV/K level below the Néel transition around 250 K, which is comparable with many topological antiferromagnetic semimetals and far excels other magnetic oxides. Our analysis indicates that the coexistence of significant symmetric and antisymmetric contributions plays a key role, pointing to the importance of extracting both contributions and a new pathway to enhanced anomalous Nernst effect for transverse thermoelectrics. To find materials with large anomalous Nernst coefficients, which is useful for energy harvesting, it is common to focus on materials with large anomalous Hall coefficients. Here, Gong et al. find a material where the anomalous Nernst effect does not show the same antisymmetric behaviour as the anomalous Hall effect.
The Classical-to-Quantum Crossover in strain-induced ferroelectric transition in SrTiO$_3$ membranes
ArXiv.org · 2025-02-04
preprintOpen accessMechanical strain presents an effective control over symmetry-breaking phase transitions. In quantum paralelectric SrTiO3, strain can induce the ferroelectric transition via modification of local Ti potential landscape. However, brittle bulk materials can only withstand limited strain range (~0.1%). Taking advantage of nanoscopically-thin freestanding membranes, we demonstrated in-situ strain-induced reversible ferroelectric transition in a single freestanding SrTiO3 membranes. We measure the ferroelectric order by detecting the local anisotropy of the Ti 3d orbital using X-ray linear dichroism at the Ti-K pre-edge, while the strain is determined by X-ray diffraction. With reduced thickness, the SrTiO3 membranes remain elastic with >1% tensile strain cycles. A robust displacive ferroelectricity appears beyond a temperature-dependent critical strain. Interestingly, we discover a crossover from a classical ferroelectric transition to a quantum regime at low temperatures, which enhances strain-induced ferroelectricity. Our results offer a new opportunities to strain engineer functional properties in low dimensional quantum materials and provide new insights into the role of the ferroelectric fluctuations in quantum paraelectric SrTiO3.
Emergent tetragonality in a fundamentally orthorhombic material
Science Advances · 2024-05-23 · 12 citations
articleOpen accessSymmetry plays a key role in determining the physical properties of materials. By Neumann’s principle, the properties of a material remain invariant under the symmetry operations of the space group to which the material belongs. Continuous phase transitions are associated with a spontaneous reduction in symmetry. Less common are examples where proximity to a continuous phase transition leads to an increase in symmetry. We find signatures of an emergent tetragonal symmetry close to a charge density wave (CDW) bicritical point in a fundamentally orthorhombic material, ErTe 3 , for which the two distinct CDW phase transitions are tuned via anisotropic strain. We first establish that tension along the a axis favors an abrupt rotation of the CDW wave vector from the c to a axis and infer the presence of a bicritical point where the two continuous phase transitions meet. We then observe a divergence of the nematic elastoresistivity approaching this putative bicritical point, indicating an emergent tetragonality in the critical behavior.
Chiral Spin-Liquid-Like State in Pyrochlore Iridate Thin Films
arXiv (Cornell University) · 2024-03-10
preprintOpen accessThe pyrochlore iridates have become ideal platforms to unravel fascinating correlated and topolog?ical phenomena that stem from the intricate interplay among strong spin-orbit coupling, electronic correlations, lattice with geometric frustration, and itinerancy of the 5d electrons. The all-in-all?out antiferromagnetic state, commonly considered as the magnetic ground state, can be dramatically altered in reduced dimensionality, leading to exotic or hidden quantum states inaccessible in bulk. Here, by means of magnetotransport, resonant elastic and inelastic x-ray scattering experiments, we discover an emergent quantum disordered state in (111) Y2Ir2O7 thin films (thickness less than 30 nm) per?sisting down to 5 K, characterized by dispersionless magnetic excitations. The anomalous Hall effect observed below an onset temperature near 135 K corroborates the presence of chiral short-range spin configurations expressed in non-zero scalar spin chirality, breaking the macroscopic time-reversal symmetry. The origin of this chiral state is ascribed to the restoration of magnetic frustration on the pyrochlore lattice in lower dimensionality, where the competing exchange interactions together with enhanced quantum fluctuations suppress any long-range order and trigger spin-liquid-like behavior with degenerate ground-state manifold.
Effect of disorder on the strain-tuned charge density wave multicriticality in Pd$_x$ErTe$_3$
arXiv (Cornell University) · 2024-12-30
preprintOpen accessWe explore, through a combination of x-ray diffraction and elastoresistivity measurements, the effect of disorder on the strain-tuned charge density wave and associated multicriticality in Pd$_x$ErTe$_3$ (x = 0, 0.01, 0.02 and 0.026). We focus particularly on the behavior near the strain-tuned bicritical point that occurs in pristine ErTe$_3$ (x=0). Our study reveals that while Pd intercalation somewhat broadens the signatures of the CDW phase transitions, the line of first-order transitions at which the CDW reorients as a function of applied strain persists in the presence of disorder and still seemingly terminates at a critical point. The critical point occurs at a lower temperature and a lower strain compared to pristine ErTe$_3$. Similarly, the nematic elastoresistance of Pd$_x$ErTe$_3$, though suppressed in magnitude and broadened relative to that of ErTe$_3$, has a markedly more symmetric response around the critical point. These observations point to disorder driving a reduction in the system's electronic orthorhombicity even while the material remains irrevocably orthorhombic due to the presence of a glide plane in the crystal structure. Disorder, it would appear, reinforces the emergence of a "pseudo-tetragonal" electronic response in this fundamentally orthorhombic material.
Frequent coauthors
- 143 shared
Jong‐Woo Kim
Weatherford College
- 63 shared
Jian Liu
- 42 shared
Yongseong Choi
Argonne National Laboratory
- 42 shared
Jiun‐Haw Chu
University of Washington
- 39 shared
Lin Hao
- 37 shared
Junyi Yang
- 35 shared
D. Meyers
Oklahoma State University Oklahoma City
- 34 shared
Darrell G. Schlom
Leibniz Institute for Crystal Growth
Awards & honors
- Cannes Award
- MITX Award
- Echo Award
- Hatch Award
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