arXiv subjects

Benjamin M. Hunt

Publications and source records attributed to Benjamin M. Hunt.

10 recordsLinked to original sources

arXiv2025

Lessons from $\alpha$-RuCl3 for pursuing quantum spin liquid physics in atomically thin materials

Quantum spin liquids can arise from Kitaev magnetic interactions, and exhibit fractionalized excitations with the potential for a topological form of quantum computation. This review surveys recent experimental and theoretical progress on the pursuit of phenomena related to Kitaev magnetism in layered and exfoliatable materials, which offer numerous opportunities to apply powerful techniques from the field of atomically thin materials. We primarily focus on the antiferromagnetic Mott insulator $\alpha$-RuCl3, which exhibits Kitaev couplings and is readily exfoliated to single- or few-layer sheets, and thus serves as a test bed for developing probes of Kitaev phenomena in atomically thin materials and devices. We introduce the Kitaev model and how it is realized in $\alpha$-RuCl3 and other material candidates; and cover $\alpha$-RuCl3 synthesis and fabrication into van der Waals heterostructure devices. A key discovery is a work-function-mediated charge transfer that heavily dopes both the $\alpha$-RuCl3 and proximate materials, and can enhance Kitaev interactions by up to 50%. We further discuss a wide range of recent results in electronic transport and optical and tunneling spectroscopies of $\alpha$-RuCl3 devices. The experimental techniques and theoretical insights developed for $\alpha$-RuCl3 establish a framework for discovering and engineering superior two-dimensional Kitaev materials that may ultimately realize elusive quantum spin liquid phases.

cond-mat.str-el↗

arXiv2024

Nanoscale ferroelectric programming of van der Waals heterostructures

The ability to create superlattices in van der Waals (vdW) heterostructures via moir\'e interference heralded a new era in the science and technology of two-dimensional materials. Through precise control of the twist angle, flat bands and strongly correlated phases have been engineered. The precise twisting of vdW layers is in some sense a bottom-up approach--a single parameter can dial in a wide range of periodic structures. Here, we describe a top-down approach to engineering nanoscale potentials in vdW layers using a buried programmable ferroelectric layer. Ultra-low-voltage electron beam lithography (ULV-EBL) is used to program ferroelectric domains in a ferroelectric Al_{1-x}B_{x}N thin film through a graphene/hexagonal boron nitride (hBN) heterostructure that is transferred on top. We demonstrate ferroelectric field effects by creating a lateral p-n junction, and demonstrate spatial resolution down to 35 nm, limited by the resolution of our scanned probe characterization methods. This innovative, resist-free patterning method is predicted to achieve 10 nm resolution and enable arbitrary programming of vdW layers, opening a pathway to create new phases that are inaccessible by moir\'e techniques. The ability to "paint" different phases of matter on a single vdW "canvas" provides a wealth of new electronic and photonic functionalities.

cond-mat.mtrl-sci↗

arXiv2023

Superlattice Engineering of Topology in Massive Dirac Fermions

We show that a superlattice potential can be employed to engineer topology in massive Dirac fermions in systems such as bilayer graphene, moiré graphene-boron nitride, and transition-metal dichalcogenide (TMD) monolayers and bilayers. We use symmetry analysis to analyze band inversions to determine the Chern number $\mathscr C$ for the valence band as a function of tunable potential parameters for a class of $C_4$ and $C_3$ symmetric potentials. We present a novel method to engineer Chern number $\mathscr{C}=2$ for the valence band and show that the applied potential at minimum must have a scalar together with a non-scalar periodic part. We discover that certain forms of the superlattice potential, which may be difficult to realize in naturally occurring moiré patterns, allow for the possibility of non-trivial topological transitions. These forms may be achievable using an external superlattice potential that can be created using contemporary experimental techniques. Our work paves the way to realize the quantum Spin Hall effect (QSHE), quantum anomalous Hall effect (QAHE), and even exotic non-Abelian anyons in the fractional quantum Hall effect (FQHE).

cond-mat.mes-hall↗

arXiv2022

Tunneling Spectroscopy of Two-Dimensional Materials Based on Via Contacts

We introduce a novel planar tunneling architecture for van der Waals heterostructures based on via contacts, namely metallic contacts embedded into through-holes in hexagonal boron nitride ($h$BN). We use the via-based tunneling method to study the single-particle density of states of two different two-dimensional (2D) materials, NbSe$_2$ and graphene. In NbSe$_2$ devices, we characterize the barrier strength and interface disorder for barrier thicknesses of 0, 1 and 2 layers of $h$BN and study the dependence on tunnel-contact area down to $(44 \pm 14)^2 $ nm$^2$. For 0-layer $h$BN devices, we demonstrate a crossover from diffusive to point contacts in the small-contact-area limit. In graphene, we show that reducing the tunnel barrier thickness and area can suppress effects due to phonon-assisted tunneling and defects in the $h$BN barrier. This via-based architecture overcomes limitations of other planar tunneling designs and produces high-quality, ultra-clean tunneling structures from a variety of 2D materials.

cond-mat.mes-hall↗

arXiv2020

Direct measurement of ferroelectric polarization in a tunable semimetal

Ferroelectricity, the electrostatic counterpart to ferromagnetism, has long been thought to be incompatible with metallicity due to screening of electric dipoles and external electric fields by itinerant charges. Recent measurements, however, demonstrated signatures of ferroelectric switching in the electrical conductance of bilayers and trilayers of WTe$_2$, a semimetallic transition metal dichalcogenide with broken inversion symmetry. An especially promising aspect of this system is that the density of electrons and holes can be continuously tuned by an external gate voltage. This degree of freedom enables investigation of the interplay between ferroelectricity and free carriers, a previously unexplored regime. Here, we employ capacitive sensing in dual-gated mesoscopic devices of bilayer WTe$_2$ to directly measure the spontaneous polarization in the metallic state and quantify the effect of free carriers on the polarization in the conduction and valence bands, separately. We compare our results to a low-energy model for the electronic bands and identify the layer-polarized states that contribute to transport and polarization simultaneously. Bilayer WTe$_2$ is thus shown to be a canonical example of a ferroelectric metal and an ideal platform for exploring polar ordering, ferroelectric transitions, and applications in the presence of free carriers.

cond-mat.mes-hall↗

arXiv2020

Quantum spin Hall edge states and interlayer coupling in twisted-bilayer WTe$_2$

The quantum spin Hall (QSH) effect, characterized by topologically protected spin-polarized edge states, was recently demonstrated in monolayers of the transition metal dichalcogenide (TMD) WTe$_2$. However, the robustness of this topological protection remains largely unexplored in van der Waals heterostructures containing one or more layers of a QSH insulator. In this work, we use scanning tunneling microscopy and spectroscopy (STM/STS) to explore the topological nature of twisted bilayer (tBL) WTe$_2$ which is produce from folded monolayers, as well as, tear-and-stack fabrication. At the tBL bilayer edge, we observe the characteristic spectroscopic signature of the QSH edge state that is absent in topologically trivial as-grown bilayer. For small twist angles, a rectangular moir\'e pattern develops, which results in local modifications of the band structure. Using first principles calculations, we quantify the interactions in tBL WTe$_2$ and its topological edge states as function of interlayer distance and conclude that it is possible to tune the topology of WTe$_2$ bilayers via the twist angle as well as interlayer interactions.

cond-mat.mes-hall↗

arXiv2020

Superconducting Contact and Quantum Interference Between Two-Dimensional van der Waals and Three-Dimensional Conventional Superconductors

Two-dimensional (2D) transition-metal dichalcogenide superconductors have unique and desirable properties for integration with conventional superconducting circuits. However, fully superconducting contact must be made between the 2D material and three-dimensional (3D) superconductors in order to employ the standard microwave drive and readout of qubits in such circuits. Here, we present a method for creating zero-resistance contacts between 2D NbSe$_2$ and 3D aluminum that behave as Josephson junctions (JJs) with large effective areas compared to 3D-3D JJs. We present a model for the supercurrent flow in a 2D-3D superconducting structure by numerical solution of the Ginzburg-Landau equations and find good agreement with experiment. These results demonstrate a crucial step towards a new generation of hybrid superconducting quantum circuits.

cond-mat.supr-con↗

arXiv2020

Electrical probes of the non-Abelian spin liquid in Kitaev materials

Recent thermal-conductivity measurements evidence a magnetic-field-induced non-Abelian spin liquid phase in the Kitaev material $α$-$\mathrm{RuCl}_{3}$. Although the platform is a good Mott insulator, we propose experiments that electrically probe the spin liquid's hallmark chiral Majorana edge state and bulk anyons, including their exotic exchange statistics. We specifically introduce circuits that exploit interfaces between electrically active systems and Kitaev materials to `perfectly' convert electrons from the former into emergent fermions in the latter---thereby enabling variations of transport probes invented for topological superconductors and fractional quantum Hall states. Along the way we resolve puzzles in the literature concerning interacting Majorana fermions, and also develop an anyon-interferometry framework that incorporates nontrivial energy-partitioning effects. Our results illuminate a partial pathway towards topological quantum computation with Kitaev materials.

cond-mat.str-el↗

arXiv2019

Proximity-induced superconducting gap in the quantum spin Hall edge state of monolayer WTe$_2$

The quantum spin Hall (QSH) state was recently demonstrated in monolayers of the transition metal dichalcogenide 1T'-WTe$_2$ and is characterized by a band gap in the two-dimensional (2D) interior and helical one-dimensional (1D) edge states. Inducing superconductivity in the helical edge states would result in a 1D topological superconductor, a highly sought-after state of matter. In the present study, we use a novel dry-transfer flip technique to place atomically-thin layers of WTe$_2$ on a van der Waals superconductor, NbSe$_2$. Using scanning tunneling microscopy and spectroscopy (STM/STS), we demonstrate atomically clean surfaces and interfaces and the presence of a proximity-induced superconducting gap in the WTe$_2$ for thicknesses from a monolayer up to 7 crystalline layers. At the edge of the WTe$_2$ monolayer, we show that the superconducting gap coexists with the characteristic spectroscopic signature of the QSH edge state. Taken together, these observations provide conclusive evidence for proximity-induced superconductivity in the QSH edge state in WTe$_2$, a crucial step towards realizing 1D topological superconductivity and Majorana bound states in this van der Waals material platform.

cond-mat.mes-hall↗

arXiv2017

Tuning Ising superconductivity with layer and spin-orbit coupling in two-dimensional transition-metal dichalcogenides

Systems that simultaneously exhibit superconductivity and spin-orbit coupling are predicted to provide a route toward topological superconductivity and unconventional electron pairing, driving significant contemporary interest in these materials. Monolayer transition-metal dichalcogenide (TMD) superconductors in particular lack inversion symmetry, enforcing a spin-triplet component of the superconducting wavefunction that increases with the strength of spin-orbit coupling. In this work, we present an experimental and theoretical study of two intrinsic TMD superconductors with large spin-orbit coupling in the atomic layer limit, metallic 2H-TaS$_2$ and 2H-NbSe$_2$. For the first time in TaS$_2$, we investigate the superconducting properties as the material is reduced to a monolayer and show that high-field measurements point to the largest upper critical field thus reported for an intrinsic TMD superconductor. In few-layer samples, we find that the enhancement of the upper critical field is sustained by the dominance of spin-orbit coupling over weak interlayer coupling, providing additional platforms for unconventional superconducting states in two dimensions.

cond-mat.supr-con↗