arXiv subjects

Mark E. Barber

Publications and source records attributed to Mark E. Barber.

15 recordsLinked to original sources

arXiv2024

Local probe of bulk and edge states in a fractional Chern insulator

Fractional quantum Hall effect (FQHE) is a prime example of topological quantum many-body phenomena, arising from the interplay between strong electron correlation, topological order, and time reversal symmetry breaking. Recently, a lattice analog of FQHE at zero magnetic field has been observed, confirming the existence of a zero-field fractional Chern insulator (FCI). Despite this, the bulk-edge correspondence -- a hallmark of FCI featuring an insulating bulk with conductive edges -- has not been directly observed. In fact, this correspondence has not been visualized in any system for fractional states due to experimental challenges. Here we report the imaging of FCI edge states in twisted MoTe2 by employing a newly developed modality of microwave-impedance microscopy. By tuning the carrier density, we observe the system evolving between metallic and FCI states, the latter of which exhibits insulating bulk and conductive edges as expected from bulk-boundary correspondence. We also observe the evolution of edge states across the topological phase transition from an incompressible Chern insulator state to a metal and finally to a putative charge ordered insulating state as a function of interlayer electric field. The local measurement further reveals tantalizing prospects of neighboring domains with different fractional orders. These findings pave the way for research into topologically protected 1D interfaces between various anyonic states at zero magnetic field, such as topological entanglement entropy, Halperin-Laughlin interfaces, and the creation of non-abelian anyons.

cond-mat.str-el↗

arXiv2024

Characterization of two fast-turnaround dry dilution refrigerators for scanning probe microscopy

Low-temperature scanning probe microscopes (SPMs) are critical for the study of quantum materials and quantum information science. Due to the rising costs of helium, cryogen-free cryostats have become increasingly desirable. However, they typically suffer from comparatively worse vibrations than cryogen-based systems, necessitating the understanding and mitigation of vibrations for SPM applications. Here we demonstrate the construction of two cryogen-free dilution refrigerator SPMs with minimal modifications to the factory default and we systematically characterize their vibrational performance. We measure the absolute vibrations at the microscope stage with geophones, and use both microwave impedance microscopy and a scanning single electron transistor to independently measure tip-sample vibrations. Additionally, we implement customized filtering and thermal anchoring schemes, and characterize the cooling power at the scanning stage and the tip electron temperature. This work serves as a reference to researchers interested in cryogen-free SPMs, as such characterization is not standardized in the literature or available from manufacturers.

cond-mat.mes-hall↗

arXiv2023

Harnessing excitons at the nanoscale -- photoelectrical platform for quantitative sensing and imaging

Excitons -- quasiparticles formed by the binding of an electron and a hole through electrostatic attraction -- hold promise in the fields of quantum light confinement and optoelectronic sensing. Atomically thin transition metal dichalcogenides (TMDs) provide a versatile platform for hosting and manipulating excitons, given their robust Coulomb interactions and exceptional sensitivity to dielectric environments. In this study, we introduce a cryogenic scanning probe photoelectrical sensing platform, termed exciton-resonant microwave impedance microscopy (ER-MIM). ER-MIM enables ultra-sensitive probing of exciton polarons and their Rydberg states at the nanoscale. Utilizing this technique, we explore the interplay between excitons and material properties, including carrier density, in-plane electric field, and dielectric screening. Furthermore, we employ deep learning for automated data analysis and quantitative extraction of electrical information, unveiling the potential of exciton-assisted nano-electrometry. Our findings establish an invaluable sensing platform and readout mechanism, advancing our understanding of exciton excitations and their applications in the quantum realm.

cond-mat.mes-hall↗

arXiv2023

Spin skyrmion gaps as signatures of strong-coupling insulators in magic-angle twisted bilayer graphene

The flat electronic bands in magic-angle twisted bilayer graphene (MATBG) host a variety of correlated insulating ground states, many of which are predicted to support charged excitations with topologically non-trivial spin and/or valley skyrmion textures. However, it has remained challenging to experimentally address their ground state order and excitations, both because some of the proposed states do not couple directly to experimental probes, and because they are highly sensitive to spatial inhomogeneities in real samples. Here, using a scanning single-electron transistor, we observe thermodynamic gaps at even integer moiré filling factors at low magnetic fields. We find evidence of a field-tuned crossover from charged spin skyrmions to bare particle-like excitations, suggesting that the underlying ground state belongs to the manifold of strong-coupling insulators. From the spatial dependence of these states and the chemical potential variation within the flat bands, we infer a link between the stability of the correlated ground states and local twist angle and strain. Our work advances the microscopic understanding of the correlated insulators in MATBG and their unconventional excitations.

cond-mat.mes-hall↗

arXiv2023

Probing momentum-dependent scattering in uniaxially stressed Sr$_2$RuO$_4$ through the Hall effect

Under in-plane uniaxial stress, the largest Fermi surface sheet of the correlated metal Sr$_2$RuO$_4$ undergoes a Lifshitz transition from an electron-like to an open geometry. We investigate the effects of this transition on transport through measurement of the longitudinal resistivity $ρ_{xx}$ and the Hall coefficient $R_\text{H}$. At temperatures where scattering is dominated by electron-electron scattering, $R_\text{H}$ becomes more negative across the Lifshitz transition, opposite to expectations from the change in Fermi surface topology. We show that this change in $R_\text{H}$ is explainable only if scattering changes throughout the Brillouin zone, not just at the point in $k$-space where the Lifshitz transition occurs. In a model of orbital-dependent scattering, the electron-electron scattering rate on sections of Fermi surface with $xy$ orbital weight decreases dramatically. On the other hand, at temperatures where defect scattering dominates $ρ_{xx}$ and $R_\text{H}$ are essentially constant across the Lifshitz transition.

cond-mat.str-el↗

arXiv2021

Correlated Hofstadter Spectrum and Flavor Phase Diagram in Magic Angle Graphene

In magic angle twisted bilayer graphene (MATBG), the moiré superlattice potential gives rise to narrow electronic bands1 which support a multitude of many-body quantum phases. Further richness arises in the presence of a perpendicular magnetic field, where the interplay between moiré and magnetic length scales leads to fractal Hofstadter subbands. In this strongly correlated Hofstadter platform, multiple experiments have identified gapped topological and correlated states, but little is known about the phase transitions between them in the intervening compressible regimes. Here, using a scanning single-electron transistor microscope to measure local electronic compressibility, we simultaneously unveil novel sequences of broken-symmetry Chern insulators (CIs) and resolve sharp phase transitions between competing states with different topological quantum numbers and spin/valley flavor occupations. Our measurements provide a complete experimental mapping of the energy spectrum and thermodynamic phase diagram of interacting Hofstadter subbands in MATBG. In addition, we observe full lifting of the degeneracy of the zeroth Landau levels (zLLs) together with level crossings, indicating moiré valley splitting. We propose a unified flavor polarization mechanism to understand the intricate interplay of topology, interactions, and symmetry breaking as a function of density and applied magnetic field in this system.

cond-mat.str-el↗

arXiv2021

Suppression of superconductivity by charge density wave order in YBa$_2$Cu$_3$O$_{6.67}$

Hole-doped cuprate superconductors show a ubiquitous tendency towards charge order. Although onset of superconductivity is known to suppress charge order, there has not so far been a decisive demonstration of the reverse process, namely, the effect of charge order on superconductivity. To gain such information, we report here the dependence of the critical temperature $T_{\mathrm{c}}$ of YBa$_2$Cu$_3$O$_{6.67}$ on in-plane uniaxial stress up to 2 GPa. At a compression of about 1 GPa along the $a$ axis, 3D-correlated charge density wave (3D CDW) order appears. We find that $T_{\mathrm{c}}$ decreases steeply as the applied stress crosses 1 GPa, showing that the appearance of 3D CDW order strongly suppresses superconductivity. Through the elastocaloric effect we resolve the heat capacity anomaly at $T_{\mathrm{c}}$, and find that it does not change drastically as the 3D CDW onsets, which shows that the condensation energy of the 3D CDW is considerably less than that of the superconductivity.

cond-mat.supr-con↗

arXiv2020

Rigid platform for applying large tunable strains to mechanically delicate samples

Response to uniaxial stress has become a major probe of electronic materials. Tuneable uniaxial stress may be applied using piezoelectric actuators, and so far two methods have been developed to couple samples to actuators. In one, actuators apply force along the length of a free, beam-like sample, allowing very large strains to be achieved. In the other, samples are affixed directly to piezoelectric actuators, allowing study of mechanically delicate materials. Here, we describe an approach that merges the two: thin samples are affixed to a substrate, that is then pressurized uniaxially using piezoelectric actuators. Using this approach, we demonstrate application of large elastic strains to mechanically delicate samples: the van der Waals-bonded material FeSe, and a sample of CeAuSb$_2$ that was shaped with a focused ion beam.

physics.app-ph↗

arXiv2020

Split superconducting and time-reversal symmetry-breaking transitions, and magnetic order in Sr$_2$RuO$_4$ under uniaxial stress

Among unconventional superconductors, Sr$_2$RuO$_4$ has become a benchmark for experimentation and theoretical analysis because its normal-state electronic structure is known with exceptional precision, and because of experimental evidence that its superconductivity has, very unusually, a spontaneous angular momentum, i.e. a chiral state. This hypothesis of chirality is however difficult to reconcile with recent evidence on the spin part of the order parameter. Measurements under uniaxial stress offer an ideal way to test for chirality, because under uniaxial stress the superconducting and chiral transitions are predicted to split, allowing the empirical signatures of each to be identified separately. Here, we report zerofield muon spin relaxation (ZF-$μ$SR) measurements on crystals placed under uniaxial stresses of up to 1.05 GPa. We report a clear stress-induced splitting between the onset temperatures of superconductivity and time-reversal symmetry breaking, consistent with qualitative expectations for chiral superconductivity. We also report the appearance of unexpected bulk magnetic order under a uniaxial stress of ~ 1.0 GPa in clean Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2019

A key role of correlation effects in the Lifshitz transition in Sr$_2$RuO$_4$

Uniaxial pressure applied along an Ru-Ru bond direction induces an elliptical distortion of the largest Fermi surface of Sr$_2$RuO$_4$, eventually causing a Fermi surface topological transition, also known as a Lifshitz transition, into an open Fermi surface. There are various anomalies in low-temperature properties associated with this transition, including maxima in the superconducting critical temperature and in resistivity. In the present paper, we report new measurements, employing new uniaxial stress apparatus and new measurements of the low-temperature elastic moduli, of the strain at which this Lifshitz transition occurs: a longitudinal strain $\varepsilon_{xx}$ of $(-0.44\pm0.06)\cdot10^{-2}$, which corresponds to a B$_{1g}$ strain $\varepsilon_{xx} - \varepsilon_{yy}$ of $(-0.66\pm0.09)\cdot10^{-2}$. This is considerably smaller than the strain corresponding to a Lifshitz transition in density functional theory calculations, even if the spin-orbit coupling is taken into account. Using dynamical mean-field theory we show that electronic correlations reduce the critical strain. It turns out that the orbital anisotropy of the local Coulomb interaction on the Ru site is furthermore important to bring this critical strain close to the experimental number, and thus well into the experimentally accessible range of strains.

cond-mat.str-el↗

arXiv2018

Piezoelectric-based uniaxial pressure cell with integrated force and displacement sensors

We present a design for a piezoelectric-driven uniaxial stress cell suitable for use at ambient and cryogenic temperatures, and that incorporates both a displacement and a force sensor. The cell has a diameter of 46 mm and a height of 13 mm. It can apply a zero-load displacement of up to ~45 $μ$m, and a zero-displacement force of up to ~245 N. With combined knowledge of the displacement and force applied to the sample, it can quickly be determined whether the sample and its mounts remain within their elastic limits. In tests on the oxide metal Sr$_2$RuO$_4$, we found that at room temperature serious plastic deformation of the sample onset at a uniaxial stress of ~0.2 GPa, while at 5 K the sample deformation remained elastic up to almost 2 GPa. This result highlights the usefulness of in situ tuning, in which the force can be applied after cooling samples to cryogenic temperatures.

physics.app-ph↗

arXiv2018

Unconventional magneto-transport in ultrapure PdCoO2 and PtCoO2

We have studied magneto transport in the single-band, quasi-two-dimensional metals PdCoO2 and PtCoO2, which have extremely long mean free paths. We observer a strong temperature dependence of the Hall resistivity in small applied, fields, linked to a large violation of Kohler's rule in the magnetoresistance. We discuss the extent to which these observations can be accounted for by standard transport theory, and describe other possible, unconventional contributions to magnetotransport in very high purity metals.

cond-mat.mes-hall↗

arXiv2017

Resistivity in the Vicinity of a Van Hove Singularity: Sr$_2$RuO$_4$ Under Uniaxial Pressure

We report the results of a combined study of the normal state resistivity and superconducting transition temperature $T_c$ of the unconventional superconductor Sr$_2$RuO$_4$ under uniaxial pressure. There is strong evidence that as well as driving $T_c$ through a maximum at $\sim$3.5 K, compressive strains $\varepsilon$ of nearly 1 % along the crystallographic [100] axis drive the $γ$ Fermi surface sheet through a Van Hove singularity, changing the temperature dependence of the resistivity from $T^2$ above and below the transition region to $T^{1.5}$ within it. This occurs in extremely pure single crystals in which the impurity contribution to the resistivity is $<$100 n$Ω$cm, so our study also highlights the potential of uniaxial pressure as a more general probe of this class of physics in clean systems.

cond-mat.str-el↗

arXiv2016

Strong Peak in $T_c$ of Sr$_2$RuO$_4$ Under Uniaxial Pressure

We report a combined experimental and theoretical study of the dependence of the superconductivity of the unconventional superconductor Sr$_2$RuO$_4$ on anisotropic strain. Novel piezoelectric apparatus is used to apply uniaxial pressures of up to $\sim$1~GPa along a $\langle 100 \rangle$ direction ($a$-axis) of the crystal lattice. $T_c$ increases from 1.5~K in unstrained material to 3.4~K at compression by $\approx$0.6\%, then falls steeply. The $c$-axis upper critical field for the strained $T_c$ = 3.4~K material is a factor of twenty larger than that of the unstrained crystal, whereas the in-plane ($a$-axis) critical field increases by only a factor of three. First-principles electronic structure calculations give evidence that the observed maximum $T_c$ occurs at or near a Lifshitz transition when the Fermi level passes through a van Hove singularity. Finally, we perform order parameter analyses using three-band renormalization group calculations. These, combined with the unexpectedly low in-plane critical field, open the possibility that the highly strained $T_c$=3.4~K Sr$_2$RuO$_4$ has an even- rather than an odd-parity order parameter. Potential implications such as a transition at nonzero strain between odd- and even-parity order parameters are discussed.

cond-mat.supr-con↗

arXiv2014

Piezoelectric-based apparatus for strain tuning

We report the design and construction of piezoelectric-based apparatus for applying continuously tuneable compressive and tensile strains to test samples. It can be used across a wide temperature range, including cryogenic temperatures. The achievable strain is large, so far up to 0.23% at cryogenic temperatures. The apparatus is compact and compatible with a wide variety of experimental probes. In addition, we present a method for mounting high-aspect-ratio samples in order to achieve high strain homogeneity.

cond-mat.mtrl-sci↗