About

Professor John Harter is an Associate Professor in the Department of Materials at the University of California, Santa Barbara. His research focuses on quantum materials, including unconventional superconductors and strongly correlated systems, where the quantum nature of electrons results in exotic material properties. His group aims to characterize and understand these materials using optical and photoelectron spectroscopies.

Research topics

• Materials science
• Physics
• Condensed matter physics
• Quantum mechanics
• Optoelectronics

Selected publications

• Persistence of charge density wave fluctuations in the absence of long-range order in a hole-doped kagome metal

  Physical Review Materials · 2026-02-05

  preprintOpen accessSenior author

  Charge density waves in kagome metals are typically identified through long-range structural order, yet their fluctuating counterparts may play an equally important role in shaping electronic phases. Using ultrafast coherent phonon spectroscopy, the authors reveal that in hole-doped CsV₃Sb₅, strong charge density wave fluctuations persist far beyond the disappearance of static order, with picosecond correlation times. These fluctuations peak near a doping-tuned quantum phase transition that coincides with a minimum in the superconducting double dome. Their results establish fluctuating charge order as a robust and ubiquitous feature of kagome metals and highlight its potential influence on superconductivity and other emergent quantum phenomena.

  PublisherOA PDFDOI

• Picosecond Expansion in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>LaAlO</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> Resonantly Driven by Infrared-Active Phonons

  Physical Review Letters · 2025-09-12 · 3 citations

  article

  We investigate the ultrafast structural dynamics of LaAlO_{3} thin films driven by short mid-infrared laser pulses at 20 THz. Time-resolved x-ray diffraction reveals an immediate lattice expansion and an acoustic breathing mode of the film. First-principles theory and a spring-mass model identify the direct coupling between coherently driven infrared-active phonons and strain as the underlying mechanism. Time-resolved optical birefringence measurements confirm that the amplitude of this acoustic mode scales linearly with the pump fluence, in agreement with theory. Furthermore, time-resolved x-ray diffuse scattering indicates that THz excitation enhances crystallinity by inducing a nonthermal increase in structural symmetry originating from preexisting defects. These findings highlight the potential of a multimodal approach-combining time-resolved x-ray and optical measurements and first-principles theory-to elucidate and control structural dynamics in nanoscale materials.

  PublisherDOI

• Quantum decoherence by magnetic fluctuations in a magnetic topological insulator

  npj Quantum Materials · 2025-07-23 · 4 citations

  articleOpen accessSenior author

  Abstract In magnetic topological insulators, spontaneous time-reversal symmetry breaking by intrinsic magnetic order can gap the topological surface spectrum, resulting in exotic properties like axion electrodynamics, the quantum anomalous Hall effect, and other topological magnetoelectric responses. Understanding the magnetic order and its coupling to topological states is essential to harness these properties. Here, we leverage near-resonant magnetic dipole optical second harmonic generation to probe magnetic fluctuations in the candidate axion insulator EuSn 2 (As,P) 2 across its antiferromagnetic phase boundary. We observe a pronounced dimensional crossover in the quantum decoherence induced by magnetic fluctuations, whereby two-dimensional in-plane ferromagnetic correlations at high temperatures give way to three-dimensional long-range order at the Néel temperature. We also observe the breaking of rotational symmetry within the long-range-ordered antiferromagnetic state and map out the resulting spatial domain structure. More generally, we demonstrate the unique capabilities of nonlinear optical spectroscopy to study quantum coherence and fluctuations in magnetic quantum materials.

  PublisherOA PDFDOI

• Absence of phonon softening across a charge density wave transition due to quantum fluctuations

  Proceedings of the National Academy of Sciences · 2025-08-01 · 3 citations

  articleOpen accessCorresponding

  Kagome metals have emerged as a frontier in condensed matter physics due to their potential to host exotic quantum states. Among these, CsV 3 Sb 5 has attracted significant attention for the unusual coexistence of charge density wave (CDW) order and unconventional superconductivity, presenting an ideal system for exploring the emergent phenomena from the interplay of phonons, electronic fluctuations, and topological effects. The nature of CDW formation in CsV 3 Sb 5 is unconventional and has sparked considerable debate. In this study, we examine the origin of the CDW state via ab initio finite-temperature simulations of the lattice dynamics. Through a comparative study of CsV 3 Sb 5 and 2H-NbSe 2 , we demonstrate that the experimental absence of phonon softening—a hallmark of conventional CDW transition—in CsV 3 Sb 5 along with the presence of a weakly first-order transition, can be attributed to quantum zero-point atomic motion. This zero-point motion smears the free energy landscape of CDW, effectively stabilizing the pristine structure even below the CDW transition temperature. We argue that this surprising behavior could cause coexistence of pristine and CDW structures across the transition and lead to a weak first-order transition. Our predicted lattice dynamical behavior is supported by coherent phonon spectroscopy in single-crystalline CsV 3 Sb 5 . Our results provide crucial insights into the formation mechanism of CDW materials that exhibit little to no phonon softening, including cuprates, and highlight the surprising role of quantum effects in emergent properties of relatively heavy-element materials like CsV 3 Sb 5 .

  PublisherOA PDFDOI

• Absence of Phonon Softening across a Charge Density Wave Transition due to Quantum Fluctuations

  arXiv (Cornell University) · 2024-10-14

  preprintOpen access

  Kagome metals have emerged as a frontier in condensed matter physics due to their potential to host exotic quantum states. Among these, CsV3Sb5 has attracted significant attention for the unusual coexistence of charge density wave (CDW) order and superconductivity, presenting an ideal system for exploring novel electronic and phononic phenomena. The nature of CDW formation in CsV3Sb5 has sparked considerable debate. Previous studies have suggested that the underlying mechanism driving the CDW transition in CsV3Sb5 is distinct from conventional ones, such as electron-phonon coupling and Fermi surface nesting. In this study, we examine the origin of the CDW state via ab initio finite-temperature simulations of the lattice dynamics associated with CDW structures in CsV3Sb5. Through a comparative study of CsV3Sb5 and 2H-NbSe2, we demonstrate that the experimental absence of phonon softening in CsV3Sb5 and the presence of a weakly first order transition can be attributed to quantum zero-point motion of the lattice, which leads to smearing of the CDW landscape and effectively stabilizes the pristine structure even below the CDW transition temperature. We argue that this surprising behavior could cause coexistence of pristine and CDW structures across the transition and lead to a weak first-order transition. We further discuss experimental implications and use the simulation to interpret coherent phonon spectroscopy results in single crystalline CsV3Sb5. These findings not only refine our fundamental understanding of CDW transitions, but also highlight the surprising role of quantum effects in influencing macroscopic properties of relatively heavy-element materials like CsV3Sb5. Our results provide crucial insights into the formation mechanism of CDW materials that exhibit little to no phonon softening, including cuprates, aiding in the understanding of the CDW phase in quantum materials.

  PublisherOA PDFDOI

• Coexistence of antiferrodistortive and polar order in a superconducting <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>SrTi</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> film

  Physical Review Materials · 2024-05-08 · 3 citations

  articleOpen access

  Strontium titanate ($\mathrm{SrTi}{\mathrm{O}}_{3}$) can exhibit multiple orders, including superconductivity, an antiferrodistortive instability, and ferroelectricity. The cooperation or competition between these orders in samples that undergo all three transitions is of great fundamental interest. Here, we report scanning transmission electron microscopy imaging of the antiferrodistortive and ferroelectric structural distortions in a compressively strained $\mathrm{SrTi}{\mathrm{O}}_{3}$ film that was previously shown to become superconducting at \ensuremath{\sim}410 mK. The experiments are complemented by first-principles simulations. Unlike the polar ferroelectric phase, which is suppressed by dopants, the antiferrodistortive order is insensitive to the presence of the free carriers. The single-domain nature of the antiferrodistortive phase excludes any role of antiferrodistortive domain walls in the superconductivity. A previously reported low-temperature resistance anomaly is associated with the ferroelectric transition, not the antiferrodistortive transition.

  PublisherOA PDFDOI

• Effects of doping on polar order in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>SrTiO</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math> from first-principles modeling

  Physical review. B./Physical review. B · 2024-11-12

  articleOpen accessSenior authorCorresponding

  ${\mathrm{SrTiO}}_{3}$ is an incipient ferroelectric and an exceptionally dilute superconductor with a domelike dependence on carrier concentration. Stabilization of a polar phase through chemical substitution or strain significantly enhances the superconducting critical temperature, suggesting a possible connection between the polar instability and unconventional Cooper pairing. To investigate the effects of doping on the polar order in ${\mathrm{SrTiO}}_{3}$, we develop a simplified free-energy model which includes only the degrees of freedom necessary to capture the relevant physics of a doped, biaxially compressively strained system. We simulate the polar and antiferrodistortive thermal phase transitions using Monte Carlo methods for different doping levels and comment on the doping dependence of the transition temperatures and the formation of polar nanodomains. In addition, the temperature-dependent phonon spectral function is calculated using Langevin simulations to investigate the lattice dynamics of the doped system. We also examine the effects of doping on the electronic structure within the polar phase, including the density of states and band splitting. Finally, we compute the polarization dependence of the Rashba parameter and the doping dependence of the Midgal ratio, and place our results in the broader context of proposed pairing mechanisms.

  PublisherOA PDFDOI

• Quantum decoherence by magnetic fluctuations in a magnetic topological insulator

  arXiv (Cornell University) · 2024-07-03

  preprintOpen accessSenior author

  In magnetic topological insulators, spontaneous time-reversal symmetry breaking by intrinsic magnetic order can gap the topological surface spectrum, resulting in exotic properties like axion electrodynamics, the quantum anomalous Hall effect, and other topological magnetoelectric responses. Understanding the magnetic order and its coupling to topological states is essential to harness these properties. Here, we leverage near-resonant magnetic dipole optical second harmonic generation to probe magnetic fluctuations in the candidate axion insulator EuSn$_2$(As,P)$_2$ across its antiferromagnetic phase boundary. We observe a pronounced dimensional crossover in the quantum decoherence induced by magnetic fluctuations, whereby two-dimensional in-plane ferromagnetic correlations at high temperatures give way to three-dimensional long-range order at the Néel temperature. We also observe the breaking of rotational symmetry within the long-range-ordered antiferromagnetic state and map out the resulting spatial domain structure. More generally, we demonstrate the unique capabilities of nonlinear optical spectroscopy to study quantum coherence and fluctuations in magnetic quantum materials.

  PublisherOA PDFDOI

• Picosecond expansion in LaAlO3 resonantly driven by infrared-active phonons

  arXiv (Cornell University) · 2024-12-22

  preprintOpen access

  We investigate the ultrafast structural dynamics of LaAlO3 thin films driven by short mid-infrared laser pulses at 20 THz. Time-resolved X-ray diffraction reveals an immediate lattice expansion and an acoustic breathing mode of the film. First-principles theory and a spring-mass model identify the direct coupling between coherently driven infrared-active phonons and strain as the underlying mechanism. Time-resolved optical birefringence measurements confirm that the amplitude of this acoustic mode scales linearly with the pump fluence, which agrees with the theory. Furthermore, time-resolved X-ray diffuse scattering indicates that THz excitation enhances crystallinity by inducing a non-thermal increase in structural symmetry originating from preexisting defects. These findings highlight the potential of a multimodal approach-combining time-resolved X-ray and optical measurements and first-principles theory-to elucidate and control structural dynamics in nanoscale materials.

  PublisherOA PDFDOI

• Picosecond volume expansion drives a later-time insulator–metal transition in a nano-textured Mott insulator

  Nature Physics · 2024-02-09 · 16 citations

  articleOpen access
  PublisherOA PDFDOI

Recent grants

• CAREER: Search for Odd-Parity Superconductivity through Proximate Polar Phases

  NSF · $716k · 2022–2026

Frequent coauthors

• Darrell G. Schlom

  Leibniz Institute for Crystal Growth

  34 shared

• Kyle Shen

  Cornell University

  31 shared

• Daniel Shai

  Cornell University

  27 shared

• Eric Monkman

  23 shared

• David Hsieh

  California Institute of Technology

  15 shared

• Carolina Adamo

  Northrop Grumman (United States)

  15 shared

• Dawei Shen

  15 shared

• Stephen D. Wilson

  University of California, Santa Barbara

  14 shared