About

Aharon Kapitulnik is the Theodore and Sydney Rosenberg Professor of Applied Physics and Physics at Stanford University. His research interests include condensed matter physics, specifically the physics of low-dimensional systems, disordered and strongly correlated electron systems, superconductivity, magnetism, and quantum phase transitions. Additionally, he works in nanoscience and quantum engineering, focusing on micro- and nano-mechanical devices, tests of gravity at sub-millimeter length scales, Casimir forces at macroscopic lengths, and magneto-optics and near-field magneto-optics.

Research topics

• Condensed matter physics
• Materials science
• Physics
• Particle physics
• Optics
• Nuclear physics
• Business

Selected publications

• Optical control of orbital magnetism in magic-angle twisted bilayer graphene

  Nature Physics · 2026-01-01

  articleSenior author
  PublisherDOI

• Possible unconventional surface superconductivity in the half-Heusler compound YPtBi

  Physical review. B./Physical review. B · 2025-05-30 · 2 citations

  articleSenior author

  We report an extensive study of the noncentrosymmetric half-Heusler superconductor YPtBi, revealing an unusual relation between bulk superconductivity and the possible appearance of surface superconductivity on the (111) oriented surface, at temperatures up to three times the bulk transition temperature. Transport measurements confirmed the low carrier density of the material and its bulk superconducting transition, which was also observed in ac susceptibility through mutual inductance (MI) measurements. However, a weak signature of superconductivity in the MI measurements appeared much above the bulk transition temperature, which was further observed in scanning tunneling spectroscopy, pointing to a possible surface superconducting state. Polar Kerr effect measurements suggest that while the bulk superconductor may exhibit an unusual nodal superconducting state, only the surface state breaks time reversal symmetry. Complementary tunneling measurements on LuPtBi are used to establish the observations on YPtBi, while density-functional theory calculations may shed light on the origin of this unusual surface state.

  PublisherDOI

• Effect of disorder on the strain-tuned charge density wave multicriticality in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Pd</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi>ErTe</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>

  Physical review. B./Physical review. B · 2025-08-06

  article

  Symmetry 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.

  PublisherDOI

• Elastocaloric evidence for a multicomponent superconductor stabilized within the nematic state in Ba(Fe _{1− <i>x</i>} Co _<i>x</i> ) ₂ As ₂

  Proceedings of the National Academy of Sciences · 2025-09-08 · 3 citations

  articleOpen access

  The 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.

  PublisherDOI

• Gapless superconductivity from low-frequency electrodynamic response of two-dimensional granular composites

  Physical Review Research · 2025-02-20 · 2 citations

  articleOpen accessSenior author

  We measured the full complex ac conductance of two-dimensional granular <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>In</a:mi><a:mo>/</a:mo><a:msub><a:mi>InO</a:mi><a:mi>x</a:mi></a:msub></a:mrow></a:math> composites using the mutual inductance technique to explore the transition from a “failed superconductor turned anomalous metal” to a robust superconductor. In this system, room-temperature annealing was adopted to tune the <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>InO</b:mi><b:mi>x</b:mi></b:msub></b:math>-mediated coupling between In grains, allowing for the observation of both a “true” superconductor-to-insulator transition and the emergence of an intervening anomalous metallic state. In this paper, we show that further annealing increases the intergrain coupling, eliminating the anomalous metallic phase but at the same time preventing the emergence of strong Bose-dominated insulating phase. The complex ac conductance revealed a <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi>T</c:mi><c:mo>→</c:mo><c:mn>0</c:mn></c:mrow></c:math> finite dissipative response in a finite magnetic field, coexisting with a robust superfluid density. The anomalous power-law spectra for the dissipative response suggest quantum critical behavior as probed in the kilohertz range, and point to signatures of gapless superconductivity in our granular superconducting system.

  PublisherDOI

• Spin-glass state in nickelate superconductors

  npj Quantum Materials · 2025-08-28 · 1 citations

  articleOpen accessSenior author

  Magneto-optical measurements in La0.8Sr0.2NiO2 and Nd0.825Sr0.175NiO2 reveal an intriguing new facet of infinite-layer nickelate superconductors: the onset of spin-glass behavior at a temperature far exceeding the superconducting critical temperature Tc. This discovery sharply contrasts with copper oxide superconductors, where magnetism and superconductivity remain largely exclusive. Moreover, the magnitude and onset temperature of the polar Kerr effect in Nd0.825Sr0.175NiO2 fabricated on SrTiO3 and (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates differ dramatically, while Tc does not.

  PublisherOA PDFDOI

• Optical control of orbital magnetism in magic angle twisted bilayer graphene

  ArXiv.org · 2025-03-27

  preprintOpen accessSenior author

  Flat bands in graphene-based moiré structures host a wide range of emerging strongly correlated and topological phenomena. Optically probing and controlling them can reveal important information such as symmetry and dynamics, but have so far been challenging due to the small energy gap compared to optical wavelengths. Here, we report near infrared optical control of orbital magnetism and associated anomalous Hall effects (AHE) in a magic angle twisted bilayer graphene (MATBG) on monolayer WSe$_2$ device. We show that the properties of the AHE, such as hysteresis and amplitude, can be controlled by light near integer moiré fillings, where spontaneous ferromagnetism exists. By modulating the light helicity, we observe periodic modulation of the transverse resistance in a wide range of fillings, indicating light induced orbital magnetization through a large inverse Faraday effect. At the transition between metallic and AHE regimes, we also reveal large and random switching of the Hall resistivity, which are attributed to optical control of percolating cluster of magnetic domains. Our results open the door to optical manipulation of correlation and topology in MATBG and related structures.

  PublisherOA PDFDOI

• Emergent tetragonality in a fundamentally orthorhombic material

  Science Advances · 2024-05-23 · 12 citations

  articleOpen access

  Symmetry 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.

  PublisherOA PDFDOI

• Elastocaloric evidence for a multicomponent superconductor stabilized within the nematic state in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

  arXiv (Cornell University) · 2024-02-28

  preprintOpen access

  The iron-based high-$T_c$ superconductors exhibit rich phase diagrams with intertwined phases, including magnetism, nematicity and superconductivity. The superconducting $T_c$ 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 $T_c$ 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 $T_c$. We rule out magnetism and re-entrant 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 sub-dominant pairing instability that competes strongly with the dominant $s^\pm$ instability. Our results thus motivate a re-examination of the pairing state and its interplay with nematicity in this extensively studied iron-based superconductor, while also demonstrating the power of ECE in uncovering strain-tuned phase diagrams of quantum materials.

  PublisherOA PDFDOI

• Gapless superconductivity in the low-frequency electrodynamic response of two-dimensional granular In/InO$_x$ composites

  arXiv (Cornell University) · 2024-06-02

  preprintOpen accessSenior author

  We measured the full complex ac conductance of two-dimensional granular In/InO$_x$ composites using the mutual inductance technique to explore the transition from a "failed-superconductor-turned anomalous metal" to a robust superconductor. In this system, room-temperature annealing was adopted to tune the InO$_x$-mediated coupling between In grains, allowing for the observation of both a "true" superconductor-to-insulator transition and the emergence of an intervening anomalous metallic state. In this paper, we show that further annealing increases the inter-grain coupling, which eliminates the anomalous metallic phase, but at the same time prevent the emergence of strong Bose-dominated insulating phase. The complex ac conductance revealed a $T\to0$ saturating dissipative response in a finite magnetic field, coexisting with a robust superfluid density. The anomalous power-law spectra for the dissipative response appear to indicate quantum critical behavior proximate to a quantum superconductor to anomalous-metal transition as probed in the kilo-Hertz range, and point to signatures of gapless superconductivity in our granular superconducting system.

  PublisherOA PDFDOI

Recent grants

• Studies of Two-Dimensional Superconductors and Multilayers in Magnetic Fields

  NSF · $231k · 1995–1998

• Search for Gravity-Like Forces at Sub-100 microns Scale

  NSF · $650k · 2006–2010

• Collaborative Research: Axion Resonant InterAction DetectioN Experiment (ARIADNE) - a Continuation Proposal

  NSF · $480k · 2018–2021

• Superconductors, Insulators, Metals and Quantum Phase Transitions in Two-Dimensional Films

  NSF · $425k · 2004–2009

• Exploring Quantum Effects in Two-Dimensional Multilayered Superconductors

  NSF · $50k · 2011–2012

Frequent coauthors

• M. R. Beasley

  Intel (United States)

  97 shared

• Zhi‐Xun Shen

  Stanford University

  75 shared

• D. B. Mitzi

  73 shared

• I. R. Fisher

  69 shared

• Steven A. Kivelson

  Stanford University

  62 shared

• D. S. Dessau

  University of Colorado Boulder

  58 shared

• B. O. Wells

  University of Connecticut

  49 shared

• Alan Fang

  44 shared

Labs

• Aharon Kapitulnik's LabPI

  Not provided

Education

• Ph.D., Physics

  Stanford University

  1985

• B.S., Physics

  University of California, Berkeley

  1980