arXiv subjects

Aharon Kapitulnik

Publications and source records attributed to Aharon Kapitulnik.

At least 37 records · Page 2Linked to original sources

arXiv2021

Nanometric Turing Patterns: Morphogenesis of a Bismuth Monolayer

Turing's reaction-diffusion theory of morphogenesis has been very successful in understanding macroscopic patterns within complex objects ranging from biological systems to sand dunes. However, this mechanism was never tested against patterns that emerge at the atomic scale, where the basic ingredients are subject to constraints imposed by quantum mechanics. Here we report evidence of a Turing pattern that appears in a strained atomic bismuth monolayer assembling on the surface of NbSe$_2$ subject to interatomic interactions and respective kinetics. The narrow range of microscopic parameters reflected in numerical analysis that observe stripe patterns and domain walls with Y-shaped junctions is a direct consequence of the quantum-mechanically allowed bond-lengths and bond-angles. This is therefore the first demonstration of a dynamically formed Turing pattern at the atomic scale.

cond-mat.mes-hall↗

arXiv2021

Robust anomalous metallic states and vestiges of self duality in two-dimensional granular In-InOx composites

Many experiments investigating magnetic field tuned superconductor-insulator transition (H-SIT), often exhibit low-temperature resistance saturation, which is interpreted as an anomalous metallic phase emerging from a "failed superconductor," thus challenging conventional theory. Here we study a random granular array of indium islands grown on a gateable layer of indium-oxide. By tuning the intergrain couplings, we reveal a wide range of magnetic fields where resistance saturation is observed, under conditions of careful electromagnetic filtering and within a wide range of linear response. Exposure to external broadband noise or microwave radiation is shown to strengthen the tendency of superconductivity, where at low field a global superconducting phase is restored. Increasing magnetic field unveils an "avoided H-SIT," that exhibits granularity-induced logarithmic divergence of the resistance/conductance above/below that transition, pointing to possible vestiges of the original emergent duality observed in a true H-SIT. We conclude that anomalous metallic phase is intimately associated with inherent inhomogeneities, exhibiting robust behavior at attainable temperatures for strongly granular two-dimensional systems.

cond-mat.supr-con↗

arXiv2021

Universal bound to the amplitude of the vortex Nernst signal in superconductors

A liquid of superconducting vortices generates a transverse thermoelectric response. This Nernst signal has a tail deep in the normal state due to superconducting fluctuations. Here, we present a study of the Nernst effect in two-dimensional hetero-structures of Nb-doped strontium titanate (STO) and in amorphous MoGe. The Nernst signal generated by ephemeral Cooper pairs above the critical temperature has the magnitude expected by theory in STO. On the other hand, the peak amplitude of the vortex Nernst signal below $T_c$ is comparable in both and in numerous other superconductors despite the large distribution of the critical temperature and the critical magnetic fields. In four superconductors belonging to different families, the maximum Nernst signal corresponds to an entropy per vortex per layer of $\approx$ k$_Bln2$.

cond-mat.supr-con↗

arXiv2020

Anomalous thermal transport and violation of Wiedemann-Franz law in the critical regime of a charge density wave transition

ErTe$_3$ is studied as a model system to explore thermal transport in a layered charge density wave (CDW) material. We present data from thermal diffusivity, resistivity, and specific heat measurements: There is a sharp decrease in thermal conductivity both parallel and perpendicular to the primary CDW at the CDW transition temperature. At the same time, the resistivity changes more gradually. Correspondingly, while well above and below $T_c$, a consistent description of the thermal transport applies with essentially independent electron and phonon contributions (estimated using the Wiedemann Franz law), in the critical regime no such description is possible; the observed behavior corresponds to a strongly coupled electron-phonon critical `soup.'

cond-mat.str-el↗

arXiv2020

Source mass characterization in the ARIADNE axion experiment

The Axion Resonant InterAction Detection Experiment (ARIADNE) is a collaborative effort to search for the QCD axion using nuclear magnetic resonance (NMR), where the axion acts as a mediator of spin-dependent forces between an unpolarized tungsten source mass and a sample of polarized helium-3 gas. Since the experiment involves precision measurement of a small magnetization, it relies on limiting ordinary magnetic noise with superconducting magnetic shielding. In addition to the shielding, proper characterization of the noise level from other sources is crucial. We investigate one such noise source in detail: the magnetic noise due to impurities and Johnson noise in the tungsten source mass.

physics.ins-det↗

arXiv2020

Observation of broken inversion and chiral symmetries in the pseudogap phase in single and double layer bismuth-based cuprates

We deduce the symmetry of the pseudogap state in the single and double layer bismuth-based cuprate superconductors by measuring and analyzing their circular and linear photogalvanic responses, which are related linearly to the chirality and inversion breaking respectively of the order parameter. After separating out the trivial contribution arising from the surface where inversion symmetry is already broken, we show that both responses start below the pseudogap temperature $T^*$ and grow below it to a sizable magnitude, revealing the broken symmetries in the bulk of the crystal. Through a detailed analysis of the dependence of the signals on the angle of incidence, the polarization of the light, and the orientation of the crystal, we are able to discover that the point group symmetry below $T^*$ is limited to $mm2$ or $mm2\underline{1}$ groups. Taking into account formation of domains and previous measurements, our results narrow down the possible symmetries of the microscopic origin of the phase transition(s) at $T^*$.

cond-mat.str-el↗

arXiv2020

Sample Shape and Boundary Dependence of Measured Transverse Thermal Properties

Despite increased interest in thermal Hall measurements for the analysis of insulating quantum materials, there remains large uncertainty in such measurements due to contact misalignment. In this paper we propose that sample geometry and uncertain boundary conditions may account for uncertainty in the measurement of $D_{xy}$ or $κ_{xy}$ as well. By running simple simulations in an open source finite-element solver, we demonstrate that measured $D_{xy}$ in a thermal Hall bar can be changed by a factor of order unity in samples with similar width and length. This geometric corrective factor depends on the distinction between uniform heat flow and constant temperature boundary couplings to a bath. Sample geometry and boundary conditions can be accounted for through simulation or by using rectangular samples to make thermal Hall measurements more reliable and reproducible when the amplitude of $κ_{xy}$ is important. Finally, we propose a contactless optical method for measuring $D_{xy}$ which is insensitive to the longitudinal diffusivity pollution caused by contact misalignment.

cond-mat.mtrl-sci↗

arXiv2020

Robust superconductivity intertwined with charge density wave and disorder in Pd-intercalated ErTe$_3$

Pd-intercalated ErTe$_3$ is studied as a model system to explore the effect of "intertwined" superconducting and charge density wave (CDW) orders. Despite the common wisdom that superconductivity emerges only when CDW is suppressed, we present data from STM and AC susceptibility measurements that show no direct competition between CDW order and superconductivity. Both coexist over most of the intercalation range, with uniform superconductivity over length scales that exceed the superconducting coherence length. This is despite persisting short-range CDW order and increased scattering from the Pd intercalation. While superconductivity is insensitive to local defects in either of the bi-directional CDWs, vestiges of the Fermi-level distortions are observed in the properties of the superconducting state.

cond-mat.supr-con↗

arXiv2020

Superconductor-Insulator Transition and the Crossover to Non Equilibrium in two-dimensional Indium - Indium-Oxide composite

Magnetic-field tuned superconductor to insulator transition was observed in a novel hybrid system of granular superconducting indium, deposited on indium oxide thin film, which exhibits global superconductivity at low magnetic fields. We have used annealing to tune the coupling to lie just at the borderline where superconductivity in the underlying InOx is suppressed, which is also close to the metal-insulator transition of the InOx. The hybrid system exhibits a "giant" magnetoresistance above the H-SIT, with critical behavior that manifests the duality between Cooper pairs and vortices.

cond-mat.supr-con↗

arXiv2020

Weyl Superconductivity in UTe2

The search for a material platform for topological quantum computation has recently focused on unconventional superconductors. Such material systems, where the superconducting order parameter breaks a symmetry of the crystal point group, are capable of hosting novel phenomena, including emergent Majorana quasiparticles. Unique among unconventional superconductors is the recently discovered UTe2, where spin-triplet superconductivity emerges from a paramagnetic normal state. Although UTe2 could be considered a relative of a family of known ferromagnetic superconductors, the unique crystal structure of this material and experimentally suggested zero Curie temperature pose a great challenge to determining the symmetries, magnetism, and topology underlying the superconducting state. These emergent properties will determine the utility of UTe2 for future spintronics and quantum information applications. Here, we report observations of a non-zero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together show that the superconductivity in UTe2 is characterized by an order parameter with two components that breaks time reversal symmetry. These data allow us to place firm constraints on the symmetries of the order parameter, which strongly suggest that UTe2 is a Weyl superconductor that hosts chiral Fermi arc surface states.

cond-mat.str-el↗

arXiv2020

Thermalization and Possible Signatures of Quantum Chaos in Complex Crystalline Materials

Analyses of thermal diffusivity data on complex insulators and on strongly correlated electron systems hosted in similar complex crystal structures suggest that quantum chaos is a good description for thermalization processes in these systems, particularly in the high temperature regime where the many phonon bands and their interactions dominate the thermal transport. Here we observe that for these systems diffusive thermal transport is controlled by a universal Planckian time scale $τ\sim \hbar/k_BT$, and a unique velocity $v_E$. Specifically, $v_E \approx v_{ph} $ for complex insulators, and $v_{ph} \lesssim v_E \ll v_{F}$ in the presence of strongly correlated itinerant electrons ($v_{ph}$ and $v_F$ are the phonons and electrons velocities respectively). For the complex correlated electron systems we further show that charge diffusivity, while also reaching the Planckian relaxation bound, is largely dominated by the Fermi velocity of the electrons, hence suggesting that it is only the thermal (energy) diffusivity that describes chaos diffusivity.

cond-mat.str-el↗

arXiv2019

A Cantilever Torque Magnetometry Method for the Measurement of Hall Conductivity of Highly Resistive Samples

We present the first measurements of Hall conductivity utilizing a new torque magnetometry method designed for insulators. A Corbino disk exhibits a magnetic dipole moment proportional to Hall conductivity when voltage is applied across a test material. This magnetic dipole moment can be measured through torque magnetometry. The symmetry of this contactless technique allows for the measurement of Hall conductivity in previously inaccessible materials. Finally, a low-temperature noise bound, the lack of systematic errors on dummy devices, and a measurement of the Hall conductivity of sputtered indium tin oxide demonstrate the efficacy of the technique.

cond-mat.mes-hall↗

arXiv2019

A lower bound to the thermal diffusivity of insulators

It has been known for decades that thermal conductivity of insulating crystals becomes proportional to the inverse of temperature when the latter is comparable to or higher than the Debye temperature. This behavior has been understood as resulting from Umklapp scattering among phonons. We put under scrutiny the magnitude of the thermal diffusion constant in this regime and find that it does not fall below a threshold set by the square of sound velocity times the Planckian time ($τ_p=\hbar/k_BT$). The conclusion, based on scrutinizing the ratio in cubic crystals with high thermal resistivity, appears to hold even in glasses where Umklapp events are not conceivable. Explaining this boundary, reminiscent of a recently-noticed limit for charge transport in metals, is a challenge to theory.

cond-mat.mtrl-sci↗

arXiv2019

Thermal Diffusivity Above Mott-Ioffe-Regel Limit

We present high-resolution thermal diffusivity measurements on several near optimally doped electron- and hole-doped cuprate systems in a temperature range that passes through the Mott-Ioffe-Regel limit, above which the quasiparticle picture fails. Our primary observations are that the inverse thermal diffusivity is linear in temperature and can be fitted to $D_Q^{-1}=aT+b$. The slope $a$ is interpreted through the Planckian relaxation time $τ\approx\hbar/k_BT$ and a thermal diffusion velocity $v_B$, which is close, but larger than the sound velocity. The intercept $b$ represent a crossover diffusion constant that separates coherent from incoherent quasiparticles. These observations suggest that both phonons and electrons participate in the thermal transport, while reaching the Planckian limit for relaxation time.

cond-mat.str-el↗

arXiv2019

Disorder Induced Suppression of CDW Long Range Order: STM Study of Pd-intercalated ErTe3

Pd-intercalated ErTe3 is studied as a model system for the interplay between a bidirectional two component charge density wave (CDW) state and disorder. Using scanning tunneling microscopy (STM), we show that introducing Pd-intercalants (i.e. disorder) disrupts the long-range order of both CDW states via the creation of dislocations, which appear associated with each CDW separately. While for weak disorder both CDW states continue to coexist throughout the sample, with no "domains" of one CDW direction or another, increasing Pd concentration has a stronger effect on the secondary CDW state, manifested in higher density of dislocations. Vestiges of the two distinct CDW phases persist to intercalation levels much above where signatures of the original phase transition are totally suppressed. This study therefore presents a first look into the disruption of multiple 2D strong-coupling CDW states by the proliferation of dislocations.

cond-mat.str-el↗

arXiv2018

Two-Stage Proximity-Induced Gap-Opening in Topological Insulator - Insulating Ferromagnet (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ - EuS Bilayers

To further investigate the interplay between ferromagnetism and topological insulators, thin films of the low-carrier topological insulator (Bi$_x$Sb$_{1-x}$)$_2$Te$_3$ were deposited on the insulating ferromagnet EuS (100) in situ. AC susceptibility indicates magnetic anomalies between $T\approx30~\mathrm{K}$ and $T\approx60~\mathrm{K}$, well above the Curie temperature $T_C \approx 15~\mathrm{K}$ of EuS. When the Fermi level is close to the Dirac point and the surface state dominates the electric conduction, sharp increases in resistance with decreasing temperatures were observed concurrently with the magnetic anomalies. Positive-negative magnetoresistance crossovers were observed at the Curie temperature, which seem only to appear when the sheet resistance exceeds the Mott-Ioffe-Regel limit $h/e^2$. A two-stage gap-opening process due to magnetic proximity is proposed.

cond-mat.mtrl-sci↗

arXiv2018

Time Reversal Symmetry Breaking in the B Phase of the Heavy Fermion Superconductor PrOs$_4$Sb$_{12}$

We present polar Kerr effect measurements of the filled skutterudite superconductor PrOs$_4$Sb$_{12}$. Simultaneous AC susceptibility measurements allow us to observe the superconducting transition under the influence of heating from the optical beam. A nonzero Kerr angle $θ_K$ develops below the superconducting transition, saturating at $\sim 300$ nrad at low temperatures. This result is repeated across several measurements of multiple samples. By extrapolating the measured $θ_K(T)$ to zero optical power, we are able to show that the Kerr angle onset temperature in one set of measurements is consistent with the transition to the B phase at $T_{C2}$. We discuss the possible explanations for this result and its impact on the understanding of multi-phase and inhomogeneous superconductivity in PrOs$_4$Sb$_{12}$.

cond-mat.supr-con↗

arXiv2018

Temperature-induced inversion of the spin-photogalvanic effect in WTe$_2$ and MoTe$_2$

We investigate the generation and temperature-induced evolution of optically-driven spin photocurrents in WTe$_2$ and MoTe$_2$. By correlating the scattering-plane dependence of the spin photocurrents with the symmetry analysis, we find that a sizeable spin photocurrent can be controllably driven along the chain direction by optically exciting the system in the high-symmetry $y$-$z$ plane. Temperature dependence measurements show that pronounced variations in the spin photocurrent emerge at temperatures that coincide with the onset of anomalies in their transport and optical properties. The decreasing trend in the temperature dependence starting below 150 K is attributed to the temperature-induced Lifshitz transition. The sign inversion of the spin photocurrent, observed around 50 K in WTe$_2$ and around 120 K in MoTe$_2$, may have its origin in an interaction that involves multiple kinds of carriers.

cond-mat.mtrl-sci↗