About

Charles Ahn is the John C. Malone Professor of Applied Physics and a Professor of Mechanical Engineering and Physics at Yale University. His research group focuses on the fabrication and study of the physical properties of novel complex oxide materials, utilizing advanced growth and characterization techniques such as molecular beam epitaxy and synchrotron x-ray scattering. His current research interests include the physics and technology of multifunctional oxides, nanofabrication, electronic control of complex order parameters in correlated oxides, and the development of nonvolatile logic switches for post-CMOS computing paradigms. Dr. Ahn earned his Ph.D. from Stanford University in 1996 and has received numerous honors, including being a Fellow of the American Physical Society, the AVS Peter Mark Memorial Award, the David and Lucile Packard Fellowship in Science and Engineering, and the Alfred P. Sloan Fellowship. He has held leadership roles such as Director of the Yale SEAS Cleanroom, Director of the Yale Institute for Nanoscience and Quantum Engineering, and Director of the Yale Center for Research on Interface Structures and Phenomena, an NSF MRSEC. His work contributes significantly to the understanding and development of complex oxide materials and their applications in advanced electronic devices.

Research topics

• Computer Science
• Physics
• Materials science
• Condensed matter physics
• Quantum mechanics
• Chemistry
• Crystallography
• Optics
• Optoelectronics
• Nanotechnology
• Engineering physics

Selected publications

• Coexisting kagome and heavy fermion flat bands in YbCr6Ge6

  Nature Communications · 2026-03-19 · 2 citations

  articleOpen access

  Flat bands, electronic states with nearly dispersionless energy-momentum structure, provide fertile ground for unconventional quantum phases. Recent observations of flat bands at the Fermi level in kagome metals open the possibility of unifying topology and correlation-driven heavy-fermion physics. Here we show that topology and heavy-fermion correlations coexist in the layered kagome metal YbCr6Ge6. At high temperatures, an intrinsic kagome flat band—arising from frustrated hopping on the kagome lattice—dominates the Fermi level. Upon cooling, localized Yb 4f-states hybridize with the topological kagome flat bands, transforming this state into momentum-independent Kondo resonance states across the entire Brillouin zone. Topological analysis of the hybridization gaps reveals filling-tunable weak and strong topological Kondo-insulating regimes, and identifies a topological Dirac–Kondo semimetal. Taken together, these results identify YbCr6Ge6 as a prototype of a topological heavy-fermion system and a platform where geometric frustration, strong correlations, and topology converge, with broad implications for correlated quantum matter. The interplay between heavy fermion systems and geometric flat bands is often hindered by a scarcity of material realizations. Here, the authors report on the coexistence of geometrically frustrated flat bands and Kondo resonance states near the Fermi level in YbCr6Ge6.

  PublisherDOI

• Isotopically enriched epitaxial CaWO$_{4}$ thin films for Er$^{3+}$ spin-photon quantum interfaces

  ArXiv.org · 2026-04-27

  articleOpen accessSenior author

  Rare earth ion (REI)-doped oxide thin films are attractive for the application of quantum interconnects due to their stable optical levels and scalability$^{1-3}$. Among them, Er$^{3+}$ doped CaWO$_{4}$ is promising because it possesses narrow optical linewidth transitions and a long spin coherence time$^{4-6}$. The electron spin coherence is limited at high temperatures by paramagnetic impurities and by the presence of the 14.3% $^{183}$W nuclear spin. To further increase the spin coherence time at millikelvin temperatures, where the paramagnetic impurities are frozen out, our approach is to synthesize chemically and isotopically purified thin films as a host material. We first grow non-isotopically enriched Er$^{3+}$ doped CaWO$_{4}$ thin films, which exhibit a 214(13) MHz photoluminescence (PL) inhomogeneous linewidth, indicating the thin film has high crystalline quality. We then grow isotopically enriched CaWO$_{4}$ thin films using an isotopically purified $^{186}$WO$_{3}$ source. Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to measure the relative concentration of W isotopes. $^{183}$W, the only W isotope that has a net nuclear spin and is the major cause of spin decoherence, was at a relative abundance of 1.2%, a factor of 10 lower than natural abundance. We also observed PL emission from single ions after integrating nano-photonic devices with the thin film. These results establish isotopically engineered CaWO$_{4}$ thin films as a promising platform for future studies of nuclear-spin-limited coherence and for scalable rare-earth-ion-based quantum nanophotonic devices.

  PublisherOA PDF

• Re-entrant unconventional superconductivity induced by rare-earth substitution in Nd1-xEuxNiO2 thin films

  Nature Communications · 2026-03-05

  articleOpen accessSenior author

  High temperature superconductivity is typically associated with strong coupling and a large superconducting gap, yet these characteristics have not been demonstrated in the nickelates. Here, we provide experimental evidence that Eu substitution in the spacer layer of Nd1-xEuxNiO2 (NENO) thin films enhances the superconducting gap, driving the system toward a strong-coupling regime. This is accompanied by a magnetic-exchange-driven magnetic-field-enhanced superconductivity. We investigate the upper critical magnetic field, Hc2, and the superconducting gap of superconducting NENO thin films with x = 0.2 to 0.35. Magnetoresistance measurements reveal magnetic-field-enhanced superconductivity in NENO films. We interpret this phenomenon as a result of an interaction between magnetic Eu ions and superconducting states in the Ni dx2-y2 orbital. The upper critical magnetic field strongly violates the weak-coupling Pauli limit. Infrared spectroscopy confirms a large gap-to-Tc ratio $$2\Delta /{k}_{B}{T}_{{\rm{c}}}\simeq 5-6$$, indicating a stronger coupling pairing mechanism in NENO relative to the Sr-doped NdNiO2. The substitution of Eu in the rare-earth layer causes pronounced modifications of the superconducting gap and magnetic interactions in Nd-based nickelates, opening new pathways to engineer high-Tc superconductivity in infinite-layer nickelates. The authors provide experimental evidence that Eu substitution in the spacer layer of Nd1-xEuxNiO2 thin films enhances the superconducting gap, driving the system toward a strong-coupling regime. The Eu substitution also introduces exchange coupling between Eu 4f magnetic moments and Ni 3dx²−y² electrons, leading to magnetic-field-enhanced “re-entrant” superconductivity.

  PublisherDOI

• Isotopically enriched epitaxial CaWO$_{4}$ thin films for Er$^{3+}$ spin-photon quantum interfaces

  arXiv (Cornell University) · 2026-04-27

  preprintOpen accessSenior author

  Rare earth ion (REI)-doped oxide thin films are attractive for the application of quantum interconnects due to their stable optical levels and scalability$^{1-3}$. Among them, Er$^{3+}$ doped CaWO$_{4}$ is promising because it possesses narrow optical linewidth transitions and a long spin coherence time$^{4-6}$. The electron spin coherence is limited at high temperatures by paramagnetic impurities and by the presence of the 14.3% $^{183}$W nuclear spin. To further increase the spin coherence time at millikelvin temperatures, where the paramagnetic impurities are frozen out, our approach is to synthesize chemically and isotopically purified thin films as a host material. We first grow non-isotopically enriched Er$^{3+}$ doped CaWO$_{4}$ thin films, which exhibit a 214(13) MHz photoluminescence (PL) inhomogeneous linewidth, indicating the thin film has high crystalline quality. We then grow isotopically enriched CaWO$_{4}$ thin films using an isotopically purified $^{186}$WO$_{3}$ source. Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to measure the relative concentration of W isotopes. $^{183}$W, the only W isotope that has a net nuclear spin and is the major cause of spin decoherence, was at a relative abundance of 1.2%, a factor of 10 lower than natural abundance. We also observed PL emission from single ions after integrating nano-photonic devices with the thin film. These results establish isotopically engineered CaWO$_{4}$ thin films as a promising platform for future studies of nuclear-spin-limited coherence and for scalable rare-earth-ion-based quantum nanophotonic devices.

  PublisherDOI

• Lifetime-limited Gigahertz-frequency Mechanical Oscillators with Millisecond Coherence Times

  ArXiv.org · 2025-04-10

  preprintOpen access

  High-frequency mechanical oscillators with long coherence times are essential to realizing a variety of high-fidelity quantum sensors, transducers, and memories. However, the unprecedented coherence times needed for quantum applications require exquisitely sensitive new techniques to probe the material origins of phonon decoherence and new strategies to mitigate decoherence in mechanical oscillators. Here, we combine non-invasive laser spectroscopy techniques with materials analysis to identify key sources of phonon decoherence in crystalline media. Using micro-fabricated high-overtone bulk acoustic-wave resonators ($μ$HBARs) as an experimental testbed, we identify phonon-surface interactions as the dominant source of phonon decoherence in crystalline quartz; lattice distortion, subsurface damage, and high concentration of elemental impurities near the crystal surface are identified as the likely causes. Removal of this compromised surface layer using an optimized polishing process is seen to greatly enhance coherence times, enabling $μ$HBARs with Q-factors of > 240 million at 12 GHz frequencies, corresponding to > 6 ms phonon coherence times and record-level f-Q products. Complementary phonon linewidth and time-domain ringdown measurements, performed using a new Brillouin-based pump-probe spectroscopy technique, reveal negligible dephasing within these oscillators. Building on these results, we identify a path to > 100 ms coherence times as the basis for high-frequency quantum memories. These findings clearly demonstrate that, with enhanced control over surfaces, dissipation and noise can be significantly reduced in a wide range of quantum systems.

  PublisherOA PDFDOI

• THz carrier dynamics in $SrTiO_{3}/LaTiO_{3}$ interface two-dimensional electron gases

  arXiv (Cornell University) · 2025-03-27

  preprintOpen access

  A two-dimensional electron gas (2DEG) forms at the interface of complex oxides like $SrTiO_{3}$ (STO) and $LaTiO_{3}$ (LTO), despite each material having a low native conductivity, as a band and a Mott insulator, respectively. The interface 2DEG hosts charge carriers with moderate charge carrier density and mobility that raised interest as a material system for applications like field-effect transistors or detectors. Of particular interest is the integration of these oxide systems in silicon technology. To this end we study the carrier dynamics in a STO/LTO/STO heterostructure epitaxially grown on Si(001) both experimentally and theoretically. Linear THz spectroscopy was performed to analyze the temperature dependent charge carrier density and mobility, which was found to be in the range of $10^{12}$ $cm^2$ and 1000 $cm^2V^{-1}s^{-1}$, respectively. Pump-probe measurements revealed a very minor optical nonlinearity caused by hot carriers with a relaxation time of several 10 ps, even at low temperature. Density functional theory calculations with a Hubbard U term on ultrathin STO-capped LTO films on STO(001) show an effective mass of 0.64-0.68 $m_{e}$.

  PublisherOA PDFDOI

• Letter to Richard on His Retirement from Yale in June 2008

  WORLD SCIENTIFIC eBooks · 2025-06-22

  letter1st authorCorresponding
  PublisherDOI

• Polarization-Controlled Structural Modulation in the Single Atomic Layer at the PbZr0.2Ti0.8O3/LaNiO3 Interface

  Microscopy and Microanalysis · 2025-07-01

  article
  PublisherDOI

• Hysteretic electrical transport in BaTiO ₃ /Ba _{1– <i>x</i>} Sr _<i>x</i> TiO ₃ /Ge heterostructures

  WORLD SCIENTIFIC eBooks · 2025-10-01

  book-chapter
  PublisherDOI

• Interface-Induced Polarization and Inhibition of Ferroelectricity in Epitaxial SrTiO ₃ /Si

  WORLD SCIENTIFIC eBooks · 2025-10-01

  book-chapter
  PublisherDOI

Recent grants

• Electronic Properties of Oxide-Semiconductor Interfaces

  NSF · $226k · 2007–2010

• Coupling Multifunctional Oxides to Semiconductors

  NSF · $660k · 2013–2018

• Center for Innovative Structures and Phenomena

  NSF · $13.6M · 2011–2018

• Control and Manipulation of Polarization and Electric Fields at Complex Oxide-Semiconductor Interfaces

  NSF · $337k · 2010–2013

Frequent coauthors

• F. J. Walker

  Yale University

  412 shared

• Sohrab Ismail‐Beigi

  Yale University

  165 shared

• Ke Zou

  University of British Columbia

  83 shared

• Divine P. Kumah

  79 shared

• C. A. F. Vaz

  Paul Scherrer Institute

  77 shared

• Ankit Disa

  Cornell University

  71 shared

• Eric I. Altman

  Yale University

  57 shared

• Y. Segal

  Massachusetts Institute of Technology

  56 shared

Awards & honors

• Fellow of the American Physical Society
• AVS Peter Mark Memorial Award
• David and Lucile Packard Fellowship in Science and Engineeri…
• Alfred P. Sloan Fellowship