arXiv subjects

Yanyu Jia

Publications and source records attributed to Yanyu Jia.

15 recordsLinked to original sources

arXiv2026

Large Thermopower with Sign-Alternating Quantum Oscillations in Insulating Monolayer WTe2

The detection of Landau-level-like energy structures near the chemical potential of an insulator is essential to the search for a class of correlated electronic matter hosting charge-neutral fermions and Fermi surfaces, a long-proposed concept that remains elusive experimentally. Here we introduce and demonstrate that the magneto-thermoelectric response of a quantum insulator can reveal critical information not available via other approaches. We report the observation of large thermopower together with quantum oscillations (QOs) in the hole-doped insulating state of monolayer tungsten ditelluride (WTe2) in magnetic fields. The measured low temperature magneto-thermopower exceeds k_B/e by more than an order of magnitude, where k_B is the Boltzmann constant and e the elementary charge. This large thermopower is a characteristic of an insulating state, consistent with high resistivity. However, as the magnetic field is swept, QOs develop in the thermopower, which remarkably undergoes sign-changes that mimic the quantum characteristic of metals due to Landau quantization. The resistivity of the sample is orders of magnitude larger than the value expected from the QOs. The sign-change in the thermoelectric response directly implies the presence of a field-induced Landau-level-like structure at the chemical potential of the insulator. Neither the large thermopower nor the sign-changes can be induced by the metallic gate nearby. Our results demonstrate a new dilemma for investigating low energy excitations in correlated materials featuring mixed quantum characteristics of metals and insulators.

cond-mat.str-el↗

arXiv2026

Mechanochemical Nano-Writing of an Atomically Thin Metal

Mechanical energy accelerates many physicochemical processes, including materials syntheses that are hard to produce with thermal energy alone. However, physical understanding connecting applied mechanical forces with internal stresses and ensuing reaction mechanisms is lacking. Here we demonstrate mechanical force-enabled synthesis and nanoscale patterning to metallize a two-dimensional (2D) material, producing an atomically-thin superconducting material. Localized force applied by atomic force microscope tips to van der Waals (vdW) encapsulated stacks of 2D bilayer MoTe2 and adjacent source Pd guides 2D Pd7MoTe2 growth with 50 nm lateral resolution. Force accelerates reaction kinetics exponentially per Eyring's stress-assisted thermal activation model, reducing synthesis temperatures from ~200 °C to near-room temperature. Finite element simulations, density functional theory, and ab-initio grand canonical Monte Carlo calculations show that tip-induced compression facilitates Pd chemisorption to tensile-strained MoTe2 that converts to uniform Pd7MoTe2. This demonstrates a new, generalizable paradigm for nanoscale synthesis of quantum materials, and high-precision engineering of superconductivity.

cond-mat.mes-hall↗

arXiv2026

Qumus: Realization of An Embodied AI Quantum Material Experimentalist

While modern Large Language Models (LLMs) and agentic artificial intelligence (AI) have demonstrated transformative capabilities in digital domains, the realization of embodied AI capable of real-world scientific discovery remains a difficult frontier. The advancements are hindered by the inherent complexity of integrating high-level reasoning, multimodal information processing and real-time physical execution. Here we introduce Qumus, the first AI quantum materials experimentalist. Physically embodied within a robotic mini-laboratory, Qumus is an intelligent, multimodal, and multi-agent system designed for the creation and nano-processing of atomically thin two-dimensional (2D) materials and stacked van der Waals (vdW) structures. Qumus autonomously navigates the full scientific cycle, from hypothesis generation and protocol planning to multi-step experimental execution, result analysis and reporting, acting as an experimentalist. Markedly, the system has achieved, for the first time, the AI-creation of graphene, as well as the first AI-fabrication of complex nanodevices including atomically thin field-effect transistors via vdW stacking. Qumus excels at these tasks by demonstrating autonomous error correction and closed-loop experimentation. Our results establish a generalizable framework for self-improving embodied AI systems that learn directly from the quantum world, opening a pathway toward accelerated discovery in quantum materials, electronics and beyond.

cond-mat.mes-hall↗

arXiv2025

van der Waals Nanoreactors

Advancing the chemical synthesis of crystals is important for both fundamental research and practical applications of quantum materials. While established bulk-phase and thin-film growth methods have enabled enormous progress, synthesizing single crystals suitable for quantum electronic discoveries remains challenging for many emerging materials. Here, we introduce van der Waals (vdW) stacks as nanochemical reactors for single-crystal synthesis and demonstrate their broad applicability in growing both elemental and compound crystals at the micrometer scale. By encapsulating atomically thin reactants that are stacked compactly with inert vdW layers such as hexagonal boron nitride (hBN), we achieve nanoconfined synthesis with the resulting crystals remaining encapsulated. As proof of concept, we synthesized isolated single crystals of elemental tellurium and distinct types of Pd-Te compounds. Structural characterization, including atomic-resolution scanning transmission electron microscopy, confirms the high crystalline quality of the products. We confirm the intrinsic semiconducting gap of tellurium and observe that non-stoichiometric PdTe1-x with a significantly reduced Te content (x ~ 0.18, a regime not previously achieved) retains uniform crystallinity and exhibits superconductivity below a critical temperature of 3.8 K. This nanochemical synthesis is broadly generalizable, chip-integrable, well-suited to a wide range of processing conditions, and compatible with nanofabrication routines for constructing devices. The concept of vdW nanoreactors offers a powerful and versatile pathway to expand the accessible landscape of quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional Superconducting Phase Diagram of Monolayer WTe2

The existence of a quantum critical point (QCP) and fluctuations around it are believed to be important for understanding the phase diagram in unconventional superconductors such as cuprates, iron pnictides, and heavy fermion superconductors. However, the QCP is usually buried deep within the superconducting dome and is difficult to investigate. The connection between quantum critical fluctuations and superconductivity remains an outstanding problem in condensed matter. Here combining both electrical transport and Nernst experiments, we explicitly demonstrate the onset of superconductivity at an unconventional QCP in gate-tuned monolayer tungsten ditelluride (WTe2), with features incompatible with the conventional Bardeen-Cooper-Schrieffer (BCS) scenario. The results lead to a novel superconducting phase diagram that is distinguished from other known superconductors. Two distinct gate-tuned quantum phase transitions are observed at the ends of the superconducting dome. We find that quantum fluctuations around the QCP of the underdoped regime are essential for understanding how the monolayer superconductivity is established. The unconventional phase diagram we report here illustrates a previously unknown relation between superconductivity and QCP.

cond-mat.mes-hall↗

arXiv2024

Anomalous Superconductivity in Twisted MoTe2 Nanojunctions

Introducing superconductivity in topological materials can lead to innovative electronic phases and device functionalities. Here, we present a new strategy for quantum engineering of superconducting junctions in moire materials through direct, on-chip, and fully encapsulated 2D crystal growth. We achieve robust and designable superconductivity in Pd-metalized twisted bilayer molybdenum ditelluride (MoTe2) and observe anomalous superconducting effects in high-quality junctions across ~ 20 moire cells. Surprisingly, the junction develops enhanced, instead of weakened, superconducting behaviors, exhibiting fluctuations to a higher critical magnetic field compared to its adjacent Pd7MoTe2 superconductor. Additionally, the critical current further exhibits a striking V-shaped minimum at zero magnetic field. These features are unexpected in conventional Josephson junctions and indeed absent in junctions of natural bilayer MoTe2 created using the same approach. We discuss implications of these observations, including the possible formation of mixed even- and odd-parity superconductivity at the moire junctions. Our results also demonstrate a pathway to engineer and investigate superconductivity in fractional Chern insulators.

cond-mat.supr-con↗

arXiv2024

Superconductivity from On-Chip Metallization on 2D Topological Chalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) is a versatile class of quantum materials of interest to various fields including, e.g., nanoelectronics, optical devices, and topological and correlated quantum matter. Tailoring the electronic properties of TMDs is essential to their applications in many directions. Here, we report that a highly controllable and uniform on-chip 2D metallization process converts a class of atomically thin TMDs into robust superconductors, a property belonging to none of the starting materials. As examples, we demonstrate the introduction of superconductivity into a class of 2D air-sensitive topological TMDs, including monolayers of Td-WTe2, 1T'-MoTe2 and 2H-MoTe2, as well as their natural and twisted bilayers, metalized with an ultrathin layer of Palladium. This class of TMDs are known to exhibit intriguing topological phases ranging from topological insulator, Weyl semimetal to fractional Chern insulator. The unique, high-quality two-dimensional metallization process is based on our recent findings of the long-distance, non-Fickian in-plane mass transport and chemistry in 2D that occur at relatively low temperatures and in devices fully encapsulated with inert insulating layers. Highly compatible with existing nanofabrication techniques for van der Waals (vdW) stacks, our results offer a route to designing and engineering superconductivity and topological phases in a class of correlated 2D materials.

cond-mat.mes-hall↗

arXiv2023

A Platform for Far-Infrared Spectroscopy of Quantum Materials at Millikelvin Temperatures

Optical spectroscopy of quantum materials at ultralow temperatures is rarely explored, yet it may provide critical characterizations of quantum phases not possible using other approaches. We describe the development of a novel experimental platform that enables optical spectroscopic studies, together with standard electronic transport, of materials at millikelvin temperatures inside a dilution refrigerator. The instrument is capable of measuring both bulk crystals and micron-sized two-dimensional van der Waals materials and devices. We demonstrate the performance by implementing photocurrent-based Fourier transform infrared spectroscopy on a monolayer WTe$_2$ device and a multilayer 1T-TaS$_2$ crystal, with a spectral range available from the near-infrared to the terahertz regime and in magnetic fields up to 5 T. In the far-infrared regime, we achieve spectroscopic measurements at a base temperature as low as ~ 43 mK and a sample electron temperature of ~ 450 mK. Possible experiments and potential future upgrades of this versatile instrumental platform are envisioned.

cond-mat.mes-hall↗

arXiv2023

Evidence for Two Dimensional Anisotropic Luttinger Liquids at Millikelvin Temperatures

While Landau's Fermi liquid theory provides the standard description for two- and three-dimensional (2D/3D) conductors, the physics of interacting one-dimensional (1D) conductors is governed by the distinct Luttinger liquid (LL) theory. Can a LL-like state, in which electronic excitations are fractionalized modes, emerge in a 2D system as a stable zero-temperature phase? This long-standing question, first brought up by Anderson decades ago, is crucial in the study of non-Fermi liquids but remains unsettled. A recent experiment identified a moiré superlattice of twisted bilayer tungsten ditelluride (tWTe_2) with a small interlayer twist angle as a 2D host of the LL physics at temperatures of a few kelvins. Here we report experimental evidence for a 2D anisotropic LL state in a substantially reduced temperature regime, down to at least 50 mK, spontaneously formed in a tWTe_2 system with a twist angle of ~ 3 degree. While the system is metallic-like and nearly isotropic above 2 K, a dramatically enhanced electronic anisotropy develops in the millikelvin regime, featuring distinct transport behaviors along two orthogonal in-plane directions. In the strongly anisotropic phase, we observe transport characteristics of a 2D LL phase, i.e., the universal power law scaling behaviors in across-wire conductance and a zero-bias dip in the differential resistance along the wire direction. Our results represent a step forward in the search for stable LL physics beyond 1D and related unconventional quantum matter.

cond-mat.mes-hall↗

arXiv2023

Surface-Confined Two-Dimensional Crystal Growth on a Monolayer

Conventional vapor deposition or epitaxial growth of two-dimensional (2D) materials and heterostructures is conducted in a large chamber in which masses transport from the source to the substrate. Here we report a chamber-free, on-chip approach for growing a 2D crystalline structures directly in a nanoscale surface-confined 2D space. The method is based on a surprising discovery of a rapid, long-distance, non-Fickian transport of a uniform layer of atomically thin palladium (Pd) on a monolayer crystal of tungsten ditelluride (WTe2), at temperatures well below the known melting points of all materials involved. The resulting nanoconfined growth realizes a controlled formation of a stable new 2D crystalline material, Pd7WTe2 , when the monolayer seed is either free-standing or fully encapsulated in a van der Waals stack. The approach is generalizable and highly compatible with nanodevice fabrication, promising to expand the library of 2D materials and their functionalities.

cond-mat.mes-hall↗

arXiv2023

Unconventional Superconducting Quantum Criticality in Monolayer WTe2

The superconductor to insulator or metal transition in two dimensions (2D) provides a valuable platform for studying continuous quantum phase transitions (QPTs) and critical phenomena. Distinct theoretical models, including both fermionic and bosonic localization scenarios, have been developed, but many questions remain unsettled despite decades of research. Extending Nernst experiments down to millikelvin temperatures, we uncover anomalous quantum fluctuations and identify an unconventional superconducting quantum critical point (QCP) in a gate-tuned excitonic quantum spin Hall insulator (QSHI), the monolayer tungsten ditelluride (WTe2). The observed vortex Nernst effect reveals singular superconducting fluctuations in the resistive normal state induced by magnetic fields or temperature, even well above the transition. Near the doping-induced QCP, the Nernst signal driven by quantum fluctuations is exceptionally large in the millikelvin regime, with a coefficient of ~ 4,100 uV/KT at zero magnetic field, an indication of the proliferation of vortices. Surprisingly, the Nernst signal abruptly disappears when the doping falls below the critical value, in striking conflict with conventional expectations. This series of phenomena, which have no prior analogue, call for careful examinations of the mechanism of the QCP, including the possibility of a continuous QPT between two distinct ordered phases in the monolayer. Our experiments open a new avenue for studying unconventional QPTs and quantum critical matter.

cond-mat.mes-hall↗

arXiv2022

Evidence for a Monolayer Excitonic Insulator

The interplay between topology and correlations can generate a variety of quantum phases, many of which remain to be explored. Recent advances have identified monolayer WTe2 as a promising material for doing so in a highly tunable fashion. The ground state of this two-dimensional (2D) crystal can be electrostatically tuned from a quantum spin Hall insulator (QSHI) to a superconductor. However, much remains unknown about the gap-opening mechanism of the insulating state. Here we report evidence that the QSHI is also an excitonic insulator (EI), arising from the spontaneous formation of electron-hole bound states (excitons). We reveal the presence of an intrinsic insulating state at the charge neutrality point (CNP) in clean samples and confirm the correlated nature of this charge-neutral insulator by tunneling spectroscopy. We provide evidence against alternative scenarios of a band insulator or a localized insulator and support the existence of an EI phase in the clean limit. These observations lay the foundation for understanding a new class of correlated insulators with nontrivial topology and identify monolayer WTe2 as a promising candidate for exploring quantum phases of ground-state excitons.

cond-mat.mes-hall↗

arXiv2021

One-Dimensional Luttinger Liquids in a Two-Dimensional Moiré Lattice

The Luttinger liquid (LL) model of one-dimensional (1D) electronic systems provides a powerful tool for understanding strongly correlated physics including phenomena such as spin-charge separation. Substantial theoretical efforts have attempted to extend the LL phenomenology to two dimensions (2D), especially in models of closely packed arrays of 1D quantum wires, each being described as a LL. Such coupled-wire models have been successfully used to construct 2D anisotropic non-Fermi liquids, quantum Hall states, topological phases, and quantum spin liquids. However, an experimental demonstration of high-quality arrays of 1D LLs suitable for realizing these models remains absent. Here we report the experimental realization of 2D arrays of 1D LLs with crystalline quality in a moiré superlattice made of twisted bilayer tungsten ditelluride (tWTe$_{2}$). Originating from the anisotropic lattice of the monolayer, the moiré pattern of tWTe$_{2}$ hosts identical, parallel 1D electronic channels, separated by a fixed nanoscale distance, which is tunable by the interlayer twist angle. At a twist angle of ~ 5 degrees, we find that hole-doped tWTe$_{2}$ exhibits exceptionally large transport anisotropy with a resistance ratio of ~ 1000 between two orthogonal in-plane directions. The across-wire conductance exhibits power-law scaling behaviors, consistent with the formation of a 2D anisotropic phase that resembles an array of LLs. Our results open the door for realizing a variety of correlated and topological quantum phases based on coupled-wire models and LL physics.

cond-mat.mes-hall↗

arXiv2021

Landau Quantization and Highly Mobile Fermions in an Insulator

In strongly correlated materials, quasiparticle excitations can carry fractional quantum numbers. An intriguing possibility is the formation of fractionalized, charge-neutral fermions, e.g., spinons and fermionic excitons, that result in neutral Fermi surfaces and Landau quantization in an insulator. While previous experiments in quantum spin liquids, topological Kondo insulators, and quantum Hall systems have hinted at charge-neutral Fermi surfaces, evidence for their existence remains far from conclusive. Here we report experimental observation of Landau quantization in a two dimensional (2D) insulator, i.e., monolayer tungsten ditelluride (WTe$_{2}$), a large gap topological insulator. Using a detection scheme that avoids edge contributions, we uncover strikingly large quantum oscillations in the monolayer insulator's magnetoresistance, with an onset field as small as ~ 0.5 tesla. Despite the huge resistance, the oscillation profile, which exhibits many periods, mimics the Shubnikov-de Haas oscillations in metals. Remarkably, at ultralow temperatures the observed oscillations evolve into discrete peaks near 1.6 tesla, above which the Landau quantized regime is fully developed. Such a low onset field of quantization is comparable to high-mobility conventional two-dimensional electron gases. Our experiments call for further investigation of the highly unusual ground state of the WTe$_{2}$ monolayer. This includes the influence of device components and the possible existence of mobile fermions and charge-neutral Fermi surfaces inside its insulating gap.

cond-mat.mes-hall↗

arXiv2019

High mobility in a van der Waals layered antiferromagnetic metal

Magnetic van der Waals (vdW) materials have been heavily pursued for fundamental physics as well as for device design. Despite the rapid advances, so far magnetic vdW materials are mainly insulating or semiconducting, and none of them possesses a high electronic mobility - a property that is rare in layered vdW materials in general. The realization of a magnetic high-mobility vdW material would open the possibility for novel magnetic twistronic or spintronic devices. Here we report very high carrier mobility in the layered vdW antiferromagnet GdTe3. The electron mobility is beyond 60,000 cm2 V-1 s-1, which is the highest among all known layered magnetic materials, to the best of our knowledge. Among all known vdW materials, the mobility of bulk GdTe3 is comparable to that of black phosphorus, and is only surpassed by graphite. By mechanical exfoliation, we further demonstrate that GdTe3 can be exfoliated to ultrathin flakes of three monolayers, and that the magnetic order and relatively high mobility is retained in approximately 20-nm-thin flakes.

cond-mat.mtrl-sci↗