arXiv subjects

Aharon Kapitulnik

Publications and source records attributed to Aharon Kapitulnik.

At least 19 recordsLinked to original sources

arXiv2026

Imaging superconducting weak spots through vortex-assisted THz near-field photovoltage

Nanoscale inhomogeneities are a defining feature of many superconducting materials, yet their local electromagnetic response has remained difficult to access experimentally. This is because their relevant energy scale lies in the terahertz range, where wavelengths -- on the order of hundreds of microns -- are too large to spatially resolve nanoscopic variations. Here, we demonstrate the first application of THz near-field photovoltage nanoscopy in a superconductor, achieving 300 nm spatial resolution at 2.52 THz. Scanning a current-biased NbN strip, we reveal photovoltage peaks within the bulk associated with nanoscopic defects of reduced superfluid density. The observed photovoltage follows the evolution of the vortex-dissipative state and is attributed to enhanced vortex-antivortex pair nucleation at defect sites. Together, these results open a direct route to probing how material inhomogeneities influence light-matter interactions in superconductors, with implications for superconducting devices and strongly inhomogeneous systems such as high-Tc and moir\'e materials.

cond-mat.supr-con↗

arXiv2026

Revealing quantum geometry effects in magic angle twisted bilayer graphene using the circular photogalvanic effect

We report a photocurrent studies of a magic angle twisted bilayer graphene device using near infrared light. Through photocurrent imaging and polarization dependence, we separate the photo-thermoelectric effect from the photogalvanic effect. We observe a circular photogalvanic effect (CPGE) over a wide range of doping and temperature. The CPGE at normal incidence constraints the symmetry of the system to C$_1$, and points to a Berry curvature dipole, in agreement with theoretical predictions for strained graphene. Remarkably, the CPGE vanishes for filling $-2.5 < \nu < -1.5$, suggesting an additional symmetry breaking in that regime. Insight into this effect is obtained through Berry curvature dipole calculations, which emphasize a novel symmetry breaking effect near $\nu=-2$.

cond-mat.str-el↗

arXiv2025

High Resolution Polar Kerr Effect Studies of Cs3Sb5 and ScV6Sn6 Below the Charge Order Transition

We report high resolution polar Kerr effect measurements on CsV3Sb5 and ScV6Sn6 single crystals in search for signatures of spontaneous polar Kerr effect (PKE) below the charge order transitions of these materials. Utilizing two separate zero-area loop Sagnac interferometers operating at 1550 nm and 830 nm wavelengths, we studied the temperature dependence of possible PKE after training with magnetic field. While a finite field Kerr measurement yielded optical rotation expected from the Pauli susceptibility of the itinerant carriers, no signal was detected at zero-field to within the noise floor limit of the apparatus of below $\sim$100 nanoradians. Simultaneous coherent reflection measurements confirm the sharpness of the charge order transition in the same optical volume as the Kerr measurements. Application of strain to reveal a hidden flux-ordered magnetic state did not result in a finite Kerr effect.

cond-mat.str-el↗

arXiv2025

Spin-glass state in nickelate superconductors

Magneto-optical measurements in La${}_{0.8}$Sr${}_{0.2}$NiO${}_2$ and Nd${}_{0.825}$Sr${}_{0.175}$NiO${}_2$ 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 $T_c$. 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 Nd${}_{0.825}$Sr${}_{0.175}$NiO${}_2$ fabricated on SrTiO${}_3$ and (LaAlO${}_3$)${}_{0.3}$(Sr${}_2$TaAlO${}_6$)${}_{0.7}$ substrates differ dramatically, while $T_c$ does not.

cond-mat.str-el↗

arXiv2025

Possible Unconventional Surface Superconductivity in the Half-Heusler YPtBi

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 3 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.

cond-mat.supr-con↗

arXiv2025

Optical control of orbital magnetism in magic angle twisted bilayer graphene

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.

cond-mat.str-el↗

arXiv2024

Effect of disorder on the strain-tuned charge density wave multicriticality in Pd$_x$ErTe$_3$

We 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.

cond-mat.str-el↗

arXiv2024

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

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.

cond-mat.supr-con↗

arXiv2024

Emergent Tetragonality in a Fundamentally Orthorhombic Material

Symmetry plays a key role in determining the physical properties of materials. By Neumann's principle, the properties of a material are 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. (For example, the onset of ferromagnetism spontaneously breaks time reversal symmetry.) Much less common are examples where proximity to a continuous phase transition leads to an increase in symmetry. Here, we find an emergent tetragonal symmetry close to an apparent charge density wave (CDW) bicritical point in a fundamentally orthorhombic material, ErTe$_3$, for which the CDW phase transitions are tuned via anisotropic strain. The underlying structure of the material remains orthorhombic for all applied strains, including at the bicritical point, due to a glide plane symmetry in the crystal structure. Nevertheless, the observation of a divergence in the anisotropy of the in-plane elastoresistivity reveals an emergent electronic tetragonality near the bicritical point.

cond-mat.mtrl-sci↗

arXiv2024

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

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.

cond-mat.supr-con↗

arXiv2023

Thermal transport measurements of the charge density wave transition in CsV$_3$Sb$_5$

We study thermalization and thermal transport in single crystals of CsV$_3$Sb$_5$ through the CDW transition by directly measuring thermal diffusivity ($D$), thermal conductivity ($κ$), resistivity ($ρ$), and specific heat ($c$). Commensurate with previous reports, we observe a sharp, narrow anomaly in specific heat associated with a first order transition that results in a CDW state below $\sim94$ K. While a corresponding sharp anomaly in thermal diffusivity is also observed, resistivity and thermal conductivity only exhibit small steps at the transition, where the feature is sharp for resistivity and broader for thermal conductivity. Scrutinizing the thermal Einstein relation $κ=cD$, we find that this relation is satisfied in the entire temperature range, except in a narrow range around the transition. The Wiedemann-Franz law seems to work outside the critical region as well. Below the transition and persisting below the two-phase regime we find strong resemblance between the resistivity anomaly and the specific heat, which may point to a secondary electronic order parameter that emerges continuously below the transition.

cond-mat.str-el↗

arXiv2023

Emergent $\mathbb{Z}_2$ symmetry near a CDW multicritical point

We consider the critical behavior associated with incommensurate unidirectional charge-density-wave ordering in a weakly orthorhombic system subject to uniaxial strain as an experimentally significant example of $U(1)\times U(1)$ multicriticality. We show that, depending on microscopic details, the phase diagram can have qualitatively different structures which can involve a vestigial meta-nematic critical point, a pair of tricritical points, a decoupled tetracritical point, or (at least at mean-field level) a bicritical point. We analyze the emergent symmetries in the critical regime and find that these can -- at least in some cases -- involve an emergent $\mathbb{Z}_2$ order parameter symmetry.

cond-mat.stat-mech↗

arXiv2023

Vortex phase diagram of kagome superconductor CsV$_3$Sb$_5$

The screening response of vortices in kagome superconductor CsV$_3$Sb$_5$ was measured using the ac mutual inductance technique. Besides confirming the absence of gapless quasiparticles in zero external magnetic field, we observe the peak effect, manifested in enhanced vortex pinning strength and critical current, in a broad intermediate range of magnetic field. The peaks are followed by another crossover from strong to weak pinning, unlike the usual peak effect that diminishes smoothly at $H_{c2}$. Hysteresis in the screening response allows the identification of a vortex glass phase which strongly correlates with the onset of the peaks. A variety of features in the temperature- and field-dependence of the screening response, corroborated by resistance and dc magnetization measurements, have allowed us to extract an $H$-$T$ phase diagram of the vortex states and to infer the irreversibility line $H_\text{irr}(T)$.

cond-mat.supr-con↗

arXiv2023

High Resolution Polar Kerr Effect Studies of CsV${}_3$Sb${}_5$: Tests for Time Reversal Symmetry Breaking Below the Charge Order Transition

We report high resolution polar Kerr effect measurements on CsV${}_3$Sb${}_5$ single crystals in search for signatures of spontaneous time reversal symmetry breaking below the charge order transition at $T^* \approx 94$ K. Utilizing two different versions of zero-area loop Sagnac interferometers operating at 1550 nm wavelength, each with the fundamental attribute that without a time reversal symmetry breaking sample at its path, the interferometer is perfectly reciprocal, we find no observable Kerr effect to within the noise floor limit of the apparatus at 30 nanoradians. Simultaneous coherent reflection ratio measurements confirm the sharpness of the charge order transition in the same optical volume as the Kerr measurements. At finite magnetic field we observe a sharp onset of a diamagnetic shift in the Kerr signal at $T^*$, which persists down to the lowest temperature without change in trend. Since 1550 nm is an energy that was shown to capture all features of the optical properties of the material that interact with the charge order transition, we are led to conclude that it is highly unlikely that time reversal symmetry is broken in the charge ordered state in CsV${}_3$Sb${}_5$.

cond-mat.str-el↗

arXiv2022

Anomalous metals: from "failed superconductor" to "failed insulator"

Resistivity saturation is found on both superconducting and insulating sides of an "avoided" magnetic-field-tuned superconductor-to-insulator transition (H-SIT) in a two-dimensional In/InO$_x$ composite, where the anomalous metallic behaviors cut off conductivity or resistivity divergence in the zero-temperature limit. The granular morphology of the material implies a system of Josephson junctions (JJ) with a broad distribution of Josephson coupling E$_J$ and charging energy E$_C$ , with a H-SIT determined by the competition between E$_J$ and E$_C$ . By virtue of self-duality across the true H-SIT, we invoke macroscopic quantum tunneling effects to explain the temperature-independent resistance where the "failed superconductor" side is a consequence of phase fluctuations and the "failed insulator" side results from charge fluctuations. While true self-duality is lost in the avoided transition, its vestiges are argued to persist, owing to the incipient duality of the percolative nature of the dissipative path in the underlying random JJ system.

cond-mat.supr-con↗

arXiv2021

Superconductor-insulator transitions in three-dimensional indium-oxide at high pressures

Experiments investigating magnetic-field-tuned superconductor-insulator transition (HSIT) mostly focus on two-dimensional material systems where the transition and its proximate ground-state phases, often exhibit features that are seemingly at odds with the expected behavior. Here we present a complementary study of a three-dimensional pressure-packed amorphous indium-oxide (InOx) powder where granularity controls the HSIT. Above a low threshold pressure of ~0.2 GPa, vestiges of superconductivity are detected, although neither a true superconducting transition nor insulating behavior are observed. Instead, a saturation at very high resistivity at low pressure is followed by saturation at very low resistivity at higher pressure. We identify both as different manifestations of anomalous metallic phases dominated by superconducting fluctuations. By analogy with previous identification of the low resistance saturation as a "failed superconductor", our data suggests that the very high resistance saturation is a manifestation of a "failed insulator". Above a threshold pressure of ~6 GPa, the sample becomes fully packed, and superconductivity is robust, with TC tunable with pressure. A quantum critical point at PC~25 GPa marks the complete suppression of superconductivity. For a finite pressure below PC, a magnetic field is shown to induce a HSIT from a true zero-resistance superconducting state to a weakly insulating behavior. Determining the critical field, HC, we show that similar to the 2D behavior, the insulating-like state maintains a superconducting character, which is quenched at higher field, above which the magnetoresistance decreases to its fermionic normal state value.

cond-mat.supr-con↗

arXiv2021

Interplay between magnetism and superconductivity in UTe2

Time-reversal symmetry breaking (TRSB) in UTe2 was inferred from observations of a spontaneous Kerr response in the superconducting state after cooling in zero magnetic field, while a finite c-axis magnetic field training was further used to determine the nature of the non-unitary composite order-parameter of this material. Here we present an extensive study of the magnetic-field-trained Kerr effect, which unveils a unique critical state of pinned ferromagnetic vortices. We show that a remanent Kerr signal that appears following the removal of a training magnetic field, which reflects the response of the TRSB order parameter and the external magnetic field through the paramagnetic susceptibility. This unambiguously demonstrate the importance of the ferromagnetic fluctuations and their intimate relation to the composite order parameter. Focusing the measurement to the center of the sample, we are able to accurately determine the maximum field that is screened by the critical state and the respective critical current. Measurements in the presence of magnetic field show the tendency of the superconductor to produce shielding currents that oppose the increase in vortex-induced magnetization due to the diverging paramagnetic susceptibility.

cond-mat.supr-con↗

arXiv2021

Emergence of Ferromagnetism Through the Metal-Insulator Transition in Undoped Indium Tin Oxide Films

We present a detailed study of the emergence of bulk ferromagnetism in low carrier density samples of undoped indium tin oxide (ITO). We used annealing to increase the density of oxygen vacancies and change sample morphology without introducing impurities through the metal insulator transition (MIT). We utilized a novel and highly sensitive "Corbino-disk torque magnetometry" technique to simultaneously measure the thermodynamic and transport effects of magnetism on the same sample after successive annealing. With increased sample granularity, carrier density increased, the sample became more metallic, and ferromagnetism appeared as resistance approached the MIT. Ferromagnetism was observed through the detection of magnetization hysteresis, anomalous Hall effect (AHE), and hysteretic magnetoresistance. A sign change of the AHE as the MIT is approached may elucidate the interplay between the impurity band and the conduction band in the weakly insulating side of the MIT.

cond-mat.mtrl-sci↗