Publications by authors named "David Mandrus"

In the presence of an external magnetic field, the Kitaev model could host either gapped topological anyons or gapless Majorana fermions. In α-RuCl_{3}, the gapped and gapless cases are only separated by a 30° rotation of the in-plane magnetic field vector. The presence or absence of the spectral gap is key for understanding the thermal transport behavior in α-RuCl_{3}.

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Surface plasmon polaritons (SPPs) provide a window into the nano-optical, electrodynamic response of their host material and its dielectric environment. Graphene/α-RuCl serves as an ideal model system for imaging SPPs since the large work function difference between these two layers facilitates charge transfer that hole dopes graphene with ∼ 10 cm free carriers. In this work, we study the emergent THz response of graphene/α-RuCl heterostructures using our home-built cryogenic scanning near-field optical microscope.

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Article Synopsis
  • Terahertz (THz) coherent phonons are being explored as effective low-energy, high-speed information carriers in advanced on-chip devices made of atomically thin materials.
  • This study focused on generating THz coherent phonons in exfoliated van der Waals (vdW) flakes of FeGeTe and FePS, discovering that the vdW flakes on a gold substrate produced significantly stronger THz phonons than those on a silicon substrate.
  • The findings were supported by frequency-domain Raman mapping and numerical simulations, which revealed that the increased surface field on the gold substrate enhances THz coherent phonon generation, suggesting a universal strategy applicable across different vdW materials.
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The spin-orbit-assisted Mott insulator α-RuCl is proximate to the coveted quantum spin liquid (QSL) predicted by the Kitaev model. In the search for the pure Kitaev QSL, reducing the dimensionality of this frustrated magnet by exfoliation has been proposed as a way to enhance magnetic fluctuations and Kitaev interactions. Here, we perform angle-dependent tunneling magnetoresistance (TMR) measurements on ultrathin α-RuCl crystals with various layer numbers to probe their magnetic, electronic, and crystal structures.

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Two-dimensional semiconductors, such as transition metal dichalcogenides, have demonstrated tremendous promise for the development of highly tunable quantum devices. Realizing this potential requires low-resistance electrical contacts that perform well at low temperatures and low densities where quantum properties are relevant. Here we present a new device architecture for two-dimensional semiconductors that utilizes a charge-transfer layer to achieve large hole doping in the contact region, and implement this technique to measure the magnetotransport properties of high-purity monolayer WSe.

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Article Synopsis
  • - Metals with a kagome lattice, like the newly discovered VSn, are interesting because they can have both flat-band and Dirac electronic structures.
  • - The study examines ScVSn, using quantum oscillations in electrical transport and magnetization to investigate its electronic properties, which align with theoretical models.
  • - Findings show a significant Berry phase for a key orbit, providing evidence of a topological band structure and enhancing understanding of the complex physics in kagome metals.
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The integration time and signal-to-noise ratio are inextricably linked when performing scanning probe microscopy based on raster scanning. This often yields a large lower bound on the measurement time, for example, in nano-optical imaging experiments performed using a scanning near-field optical microscope (SNOM). Here, we utilize sparse scanning augmented with Gaussian process regression to bypass the time constraint.

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The kagome metals display an intriguing variety of electronic and magnetic phases arising from the connectivity of atoms on a kagome lattice. A growing number of these materials with vanadium-kagome nets host charge-density waves (CDWs) at low temperatures, including ScVSn, CsVSb, and VSb. Curiously, only the Sc version of the RVSn materials with a HfFeGe-type structure hosts a CDW (R = Gd-Lu, Y, Sc).

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Article Synopsis
  • - The study explores the electronic properties of a graphene and α-ruthenium trichloride (α-RuCl) heterostructure, which may have significant implications for next-gen optoelectronic devices due to α-RuCl being a Mott insulator and Kitaev material.
  • - Using advanced techniques like photoemission spectroscopy and low-energy electron microscopy, researchers visualize charge transfer between graphene and α-RuCl, altering the electronic characteristics of both materials at their interface.
  • - The findings highlight the strong interaction between graphene and α-RuCl, suggesting potential new methods to manipulate electronic properties in 2D materials, crucial for developing low-power electronic applications.
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Unusual electrical transport properties associated with weak or strong localization are sometimes found in disordered electronic materials. Here, we report experimental observation of a crossover of electronic behavior from weak localization to enhanced weak localization due to the spatial influence of disorder induced by ZrO nanopillars in (LaSrMnO):(ZrO) (x = 0, 0.2, and 0.

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Article Synopsis
  • The study explores the use of work-function-mediated charge transfer for controlling the electrostatics of individual atomic layers, using α-RuCl as a 2D electron acceptor next to hexagonal boron nitride (BN).
  • It highlights how this arrangement induces unique nano-optical behavior in BN by causing interlayer charge polarization, resulting in a reduction of phonon polariton (PhP) propagation length significantly beyond intrinsic losses.
  • The findings are backed by advanced techniques like scattering-type scanning near-field optical microscopy and first-principles calculations, demonstrating the promising applications of charge-transfer heterostructures in enhancing the optoelectronic properties of 2D insulators.
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When a topological insulator is incorporated into a Josephson junction, the system is predicted to reveal the fractional Josephson effect with a 4π-periodic current-phase relation. Here, we report the measurement of a 4π-periodic switching current through an asymmetric SQUID, formed by the higher-order topological insulator WTe. Contrary to the established opinion, we show that a high asymmetry in critical current and negligible loop inductance are not sufficient by themselves to reliably measure the current-phase relation.

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Competition among exchange interactions is able to induce novel spin correlations on a bipartite lattice without geometrical frustration. A prototype example is the spiral spin liquid, which is a correlated paramagnetic state characterized by subdimensional degenerate propagation vectors. Here, using spectral graph theory, we show that spiral spin liquids on a bipartite lattice can be approximated by a further-neighbor model on the corresponding line-graph lattice that is nonbipartite, thus broadening the space of candidate materials that may support the spiral spin liquid phases.

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Materials hosting kagome lattices have drawn interest for the diverse magnetic and electronic states generated by geometric frustration. In the AV_{3}Sb_{5} compounds (A=K, Rb, Cs), stacked vanadium kagome layers give rise to unusual charge density waves (CDW) and superconductivity. Here we report single-crystal growth and characterization of ScV_{6}Sn_{6}, a hexagonal HfFe_{6}Ge_{6}-type compound that shares this structural motif.

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We report the magnetic and electronic transport properties of Mn-doped LaTiMnO(= 0, 0.1, 0.3, 0.

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The honeycomb magnet α-RuCl has attracted considerable interest because it is proximate to the Kitaev Hamiltonian whose excitations are Majoranas and vortices. The thermal Hall conductivity κ of Majorana fermions is predicted to be half-quantized. Half-quantization of κ/T (T, temperature) was recently reported, but this observation has proven difficult to reproduce.

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Spectrally narrow optical resonances can be used to generate slow light, i.e., a large reduction in the group velocity.

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Recently, log-periodic quantum oscillations have been detected in the topological materials zirconium pentatelluride (ZrTe) and hafnium pentatelluride (HfTe), displaying an intriguing discrete scale invariance (DSI) characteristic. In condensed materials, the DSI is considered to be related to the quasi-bound states formed by massless Dirac fermions with strong Coulomb attraction, offering a feasible platform to study the long-pursued atomic-collapse phenomenon. Here, we demonstrate that a variety of atomic vacancies in the topological material HfTe can host the geometric quasi-bound states with a DSI feature, resembling an artificial supercritical atom collapse.

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Ordered mesoscale structures in 2D materials induced by small misorientations have allowed for a wide variety of electronic, ferroelectric, and quantum phenomena to be explored. Until now, the only mechanism to induce this periodic ordering was mechanical rotations between the layers, with the periodicity of the resulting moiré pattern being directly related to twist angle. Here we report a fundamentally distinct mechanism for emergence of mesoscopic periodic patterns in multilayer sulfur-containing metal phosphorus trichalcogenide, MnPS, induced by the electron beam.

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Interlayer excitons (IXs) in MoSe-WSe heterobilayers have generated interest as highly tunable light emitters in transition metal dichalcogenide (TMD) heterostructures. Previous reports of spectrally narrow (<1 meV) photoluminescence (PL) emission lines at low temperature have been attributed to IXs localized by the moiré potential between the TMD layers. We show that spectrally narrow IX PL lines are present even when the moiré potential is suppressed by inserting a bilayer hexagonal boron nitride (hBN) spacer between the TMD layers.

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Controlling the flow of charge neutral interlayer exciton (IX) quasiparticles can potentially lead to low loss excitonic circuits. Here, we report unidirectional transport of IXs along nanoscale electrostatically defined channels in an MoSe-WSe heterostructure. These results are enabled by a lithographically defined triangular etch in a graphene gate to create a potential energy "slide".

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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. 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.

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Spin-orbit torque (SOT)-driven deterministic control of the magnetic state of a ferromagnet with perpendicular magnetic anisotropy is key to next-generation spintronic applications including non-volatile, ultrafast and energy-efficient data-storage devices. However, field-free deterministic switching of perpendicular magnetization remains a challenge because it requires an out-of-plane antidamping torque, which is not allowed in conventional spin-source materials such as heavy metals and topological insulators due to the system's symmetry. The exploitation of low-crystal symmetries in emergent quantum materials offers a unique approach to achieve SOTs with unconventional forms.

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The information content of crystalline materials becomes astronomical when collective electronic behavior and their fluctuations are taken into account. In the past decade, improvements in source brightness and detector technology at modern X-ray facilities have allowed a dramatically increased fraction of this information to be captured. Now, the primary challenge is to understand and discover scientific principles from big datasets when a comprehensive analysis is beyond human reach.

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We demonstrate ultrasharp (≲10 nm) lateral p-n junctions in graphene using electronic transport, scanning tunneling microscopy, and first-principles calculations. The p-n junction lies at the boundary between differentially doped regions of a graphene sheet, where one side is intrinsic and the other is charge-doped by proximity to a flake of α-RuCl across a thin insulating barrier. We extract the p-n junction contribution to the device resistance to place bounds on the junction width.

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