arXiv subjects

Jiaqi Cai

Publications and source records attributed to Jiaqi Cai.

35 records · Page 2Linked to original sources

arXiv2024

Direct magnetic imaging of fractional Chern insulators in twisted MoTe$_2$ with a superconducting sensor

In the absence of time reversal symmetry, orbital magnetization provides a sensitive probe of topology and interactions, with particularly rich phenomenology in Chern insulators where topological edge states carry large equilibrium currents. Here, we use a nanoscale superconducting sensor to map the magnetic fringe fields in twisted bilayers of MoTe$_2$, where transport and optical sensing experiments have revealed the formation of fractional Chern insulator (FCI) states at zero magnetic field. At a temperature of 1.6K, we observe oscillations in the local magnetic field associated with fillings $ν=-1,-2/3,-3/5,-4/7$ and $-5/9$ of the first moiré hole band, consistent with the formation of FCIs at these fillings. By quantitatively reconstructing the magnetization, we determine the local thermodynamic gaps of the most robust FCI state at $ν=-2/3$, finding $^{-2/3}Δ$ as large as 7 meV. Spatial mapping of the charge density- and displacement field-tuned magnetic phase diagram further allows us to characterize sample disorder, which we find to be dominated by both inhomogeneity in the effective unit cell area as well as inhomogeneity in the band edge offset and bound dipole moment. Our results highlight both the challenges posed by structural disorder in the study of twisted homobilayer moiré systems and the opportunities afforded by the remarkably robust nature of the underlying correlated topological states.

cond-mat.mes-hall↗

arXiv2024

Absence of Weyl nodes in EuCd$_2$As$_2$ revealed by the carrier density dependence of the anomalous Hall effect

The antiferromagnetic layered compound EuCd$_2$As$_2$ is widely considered as a leading candidate of ideal Weyl semimetal, featuring a single pair of Weyl nodes in its field-induced ferromagnetic (FM) state. Nevertheless, this view has recently been challenged by an optical spectroscopy study, which suggests that it is a magnetic semiconductor. In this study, we have successfully synthesized highly insulating EuCd$_2$As$_2$ crystals with carrier density reaching as low as $2\times 10^{15}$ $\text{cm}^{-3}$. The magneto-transport measurements revealed a progressive decrease of the anomalous Hall conductivity (AHC) by several orders of magnitude as the carrier density decreases. This behavior contradicts with what is expected from the intrinsic AHC generated by the Weyl points, which is independent of carrier density as the Fermi level approaches the charge neutrality point. In contrast, the scaling relationship between AHC and longitudinal conductivity aligns with the characteristics of variable range hopping insulators. Our results suggest that EuCd$_2$As$_2$ is a magnetic semiconductor rather than a topological Weyl semimetal.

cond-mat.mtrl-sci↗

arXiv2023

Interface-Induced Superconductivity in Magnetic Topological Insulator-Iron Chalcogenide Heterostructures

When two different electronic materials are brought together, the resultant interface often shows unexpected quantum phenomena, including interfacial superconductivity and Fu-Kane topological superconductivity (TSC). Here, we use molecular beam epitaxy (MBE) to synthesize heterostructures formed by stacking together two magnetic materials, a ferromagnetic topological insulator (TI) and an antiferromagnetic iron chalcogenide (FeTe). We discover emergent interface-induced superconductivity in these heterostructures and demonstrate the trifecta occurrence of superconductivity, ferromagnetism, and topological band structure in the magnetic TI layer, the three essential ingredients of chiral TSC. The unusual coexistence of ferromagnetism and superconductivity can be attributed to the high upper critical magnetic field that exceeds the Pauli paramagnetic limit for conventional superconductors at low temperatures. The magnetic TI/FeTe heterostructures with robust superconductivity and atomically sharp interfaces provide an ideal wafer-scale platform for the exploration of chiral TSC and Majorana physics, constituting an important step toward scalable topological quantum computation.

cond-mat.mes-hall↗

arXiv2023

Symmetry-broken Chern insulators in twisted double bilayer graphene

Twisted double bilayer graphene (tDBG) has emerged as an especially rich platform for studying strongly correlated and topological states of matter. The material features moiré bands that can be continuously deformed by both perpendicular displacement field and twist angle. Here, we construct a phase diagram representing of the correlated and topological states as a function of these parameters, based on measurements on over a dozen tDBG devices encompassing the two distinct stacking configurations in which the constituent Bernal bilayer graphene sheets are rotated either slightly away from 0° or 60°. We find a hierarchy of symmetry-broken states that emerge sequentially as the twist angle approaches an apparent optimal value of $θ\approx$ 1.34°. Among them, we discover a sequence of symmetry-broken Chern insulator (SBCI) states that arise only within a narrow range of twist angles ($\approx$ 1.33° to 1.39°). We observe an associated anomalous Hall effect at zero field in all samples supporting SBCI states, indicating spontaneous time-reversal symmetry breaking and possible moiré unit cell enlargement at zero magnetic field.

cond-mat.mes-hall↗

arXiv2023

Observation of Fractionally Quantized Anomalous Hall Effect

The integer quantum anomalous Hall (QAH) effect is a lattice analog of the quantum Hall effect at zero magnetic field. This striking transport phenomenon occurs in electronic systems with topologically nontrivial bands and spontaneous time-reversal symmetry breaking. Discovery of its putative fractional counterpart in the presence of strong electron correlations, i.e., the fractional quantum anomalous Hall (FQAH) effect, would open a new chapter in condensed matter physics. Here, we report the direct observation of both integer and fractional QAH effects in electrical measurements on twisted bilayer MoTe$_2$. At zero magnetic field, near filling factor $ν= -1$ (one hole per moiré unit cell) we see an extended integer QAH plateau in the Hall resistance $R_\text{xy}$ that is quantized to $h/e^2 \pm 0.1 \%$ while the longitudinal resistance $R_\text{xx}$ vanishes. Remarkably, at $ν=-2/3$ and $-3/5$ we see plateau features in $R_\text{xy}$ at $3h/2e^2 \pm 1\%$ and $5h/3e^2 \pm 3\%$, respectively, while $R_\text{xx}$ remains small. All these features shift linearly in an applied magnetic field with slopes matching the corresponding Chern numbers $-1$, $-2/3$, and $-3/5$, precisely as expected for integer and fractional QAH states. In addition, at zero magnetic field, $R_\text{xy}$ is approximately $2h/e^2$ near half filling ($ν= -1/2$) and varies linearly as $ν$ is tuned. This behavior resembles that of the composite Fermi liquid in the half-filled lowest Landau level of a two-dimensional electron gas at high magnetic field. Direct observation of the FQAH and associated effects paves the way for researching charge fractionalization and anyonic statistics at zero magnetic field.

cond-mat.mes-hall↗

arXiv2023

Signatures of Fractional Quantum Anomalous Hall States in Twisted MoTe2 Bilayer

The interplay between spontaneous symmetry breaking and topology can result in exotic quantum states of matter. A celebrated example is the quantum anomalous Hall (QAH) state, which exhibits an integer quantum Hall effect at zero magnetic field thanks to its intrinsic ferromagnetism. In the presence of strong electron-electron interactions, exotic fractional-QAH (FQAH) states at zero magnetic field can emerge. These states could host fractional excitations, including non-Abelian anyons - crucial building blocks for topological quantum computation. Flat Chern bands are widely considered as a desirable venue to realize the FQAH state. For this purpose, twisted transition metal dichalcogenide homobilayers in rhombohedral stacking have recently been predicted to be a promising material platform. Here, we report experimental signatures of FQAH states in 3.7-degree twisted MoTe2 bilayer. Magnetic circular dichroism measurements reveal robust ferromagnetic states at fractionally hole filled moiré minibands. Using trion photoluminescence as a sensor, we obtain a Landau fan diagram which shows linear shifts in carrier densities corresponding to the v=-2/3 and -3/5 ferromagnetic states with applied magnetic field. These shifts match the Streda formula dispersion of FQAH states with fractionally quantized Hall conductance of -2/3$e^2/h$ and -3/5$e^2/h$, respectively. Moreover, the v=-1 state exhibits a dispersion corresponding to Chern number -1, consistent with the predicted QAH state. In comparison, several non-ferromagnetic states on the electron doping side do not disperse, i.e., are trivial correlated insulators. The observed topological states can be further electrically driven into topologically trivial states. Our findings provide clear evidence of the long-sought FQAH states, putting forward MoTe2 moiré superlattices as a fascinating platform for exploring fractional excitations.

cond-mat.mes-hall↗

arXiv2023

Programming Correlated Magnetic States via Gate Controlled Moiré Geometry

Understanding quantum many-body systems is at the heart of condensed matter physics. The ability to control the underlying lattice geometry of a system, and thus its many-body interactions, would enable the realization of and transition between emergent quantum ground states. Here, we report in-situ gate switching between honeycomb and triangular lattice geometries of an electron many-body Hamiltonian in R-stacked MoTe2 moiré bilayers, resulting in switchable magnetic exchange interactions. At zero electric field, we observe a correlated ferromagnetic insulator near one hole per moiré unit cell (ν=-1), i.e., a quarter-filled honeycomb lattice, with a widely tunable Curie temperature up to 14K. Fully polarizing layer pseudospin via electric field switches the system into a half-filled triangular lattice with antiferromagnetic interactions. Further doping this layer-polarized superlattice introduces carriers into the empty layer, tuning the antiferromagnetic exchange interaction back to ferromagnetic. Our work demonstrates R-stacked MoTe2 moirés to be a new laboratory for engineering correlated states with nontrivial topology.

cond-mat.mes-hall↗

arXiv2023

Dynamically tunable moiré Rydberg excitons in a monolayer semiconductor on twisted bilayer graphene

Moiré excitons are emergent optical excitations in 2D semiconductors with deep moiré superlattice potentials. While these excitations have been realized in several platforms, a system with dynamically tunable moiré potential to tailor the moiré exciton properties is yet to be realized. Here, we present a continuously tunable moiré potential in a monolayer WSe2 that is enabled by its proximity to twisted bilayer graphene (TBG) near the magic-angle. Due to its flat electronic bands, charge distribution is highly localized and forms a triangular lattice in TBG. Tuning the local charge density via electrostatic gating, TBG thus provides a spatially varying and dynamically tunable dielectric superlattice for modulating monolayer exciton wavefunctions. By performing optical reflection spectroscopy, we observe emergent moiré exciton Rydberg branches in monolayer WSe2 with increased energy splitting upon doping TBG. The twist-angle dependence reveals that the observation is due to a hybridization between bright and dark Rydberg states enabled by the moiré potential. Further, at the magic-angle near 1.1°, the moiré Rydberg excitons form a sawtooth pattern with doping owing to the formation of strongly correlated states in the TBG. Our study provides a new platform for engineering moiré excitons as well as optical accessibility to the electronic states with small correlation gaps in TBG.

cond-mat.mes-hall↗

arXiv2023

Strain-programmable van der Waals magnetic tunnel junctions

The magnetic tunnel junction (MTJ) is a backbone device for spintronics. Realizing next generation energy efficient MTJs will require operating mechanisms beyond the standard means of applying magnetic fields or large electrical currents. Here, we demonstrate a new concept for programmable MTJ operation via strain control of the magnetic states of CrSBr, a layered antiferromagnetic semiconductor used as the tunnel barrier. Switching the CrSBr from antiferromagnetic to ferromagnetic order generates a giant tunneling magnetoresistance ratio without external magnetic field at temperatures up to ~ 140 K. When the static strain is set near the phase transition, applying small strain pulses leads to active flipping of layer magnetization with controlled layer number and thus magnetoresistance states. Further, finely adjusting the static strain to a critical value turns on stochastic switching between metastable states, with a strain-tunable sigmoidal response curve akin to the stochastic binary neuron. Our results highlight the potential of strain-programmable van der Waals MTJs towards spintronic applications, such as magnetic memory, random number generation, and probabilistic and neuromorphic computing.

cond-mat.mes-hall↗

arXiv2022

Anomalous Second Harmonic Generation from Atomically Thin MnBi2Te4

MnBi2Te4 is a van der Waals topological insulator with intrinsic intralayer ferromagnetic exchange and A-type antiferromagnetic interlayer coupling. Theoretically, it belongs to a class of structurally centrosymmetric crystals whose layered antiferromagnetic order breaks inversion symmetry for even layer numbers, making optical second harmonic generation (SHG) an ideal probe of the coupling between the crystal and magnetic structures. Here, we perform magnetic field and temperature-dependent SHG measurements on MnBi2Te4 flakes ranging from bulk to monolayer thickness. We find that the dominant SHG signal from MnBi2Te4 is unexpectedly unrelated to both magnetic state and layer number. We suggest that surface SHG is the likely source of the observed strong SHG, whose symmetry matches that of the MnBi2Te4-vacuum interface. Our results highlight the importance of considering the surface contribution to inversion symmetry-breaking in van der Waals centrosymmetric magnets.

cond-mat.mtrl-sci↗

arXiv2022

Creation of Chiral Interface Channels for Quantized Transport in Magnetic Topological Insulator Multilayer Heterostructures

One-dimensional (1D) topologically protected states are usually formed at the interface between two-dimensional (2D) materials with different topological invariants. Therefore, 1D chiral interface channels (CICs) can be created at the boundary of two quantum anomalous Hall (QAH) insulators with different Chern numbers. Such a QAH junction can function as a chiral edge current distributer at zero magnetic field, but its realization remains challenging. Here, by employing an in-situ mechanical mask, we use molecular beam epitaxy (MBE) to synthesize QAH insulator junctions, in which two QAH insulators with different Chern numbers are connected along a 1D junction. For the junction between C = 1 and C = -1 QAH insulators, we observe quantized transport and demonstrate the appearance of the two parallel propagating CICs along the magnetic domain wall at zero magnetic field. Moreover, since the Chern number of the QAH insulators in magnetic topological insulator (TI)/TI multilayers can be tuned by altering magnetic TI/TI bilayer periods, the junction between two QAH insulators with arbitrary Chern numbers can be achieved by growing different periods of magnetic TI/TI on the two sides of the sample. For the junction between C = 1 and C = 2 QAH insulators, our quantized transport shows that a single CIC appears at the interface. Our work lays down the foundation for the development of QAH insulator-based electronic and spintronic devices, topological chiral networks, and topological quantum computations.

cond-mat.mes-hall↗

arXiv2022

Crossover of Ising- to Rashba-Type Superconductivity in Epitaxial Bi2Se3/Monolayer NbSe2 Heterostructures

A topological insulator (TI) interfaced with an s-wave superconductor has been predicted to host an unusual form of superconductivity known as topological superconductivity (TSC). Molecular beam epitaxy (MBE) has been the primary approach in the scalable synthesis of the TI/superconductor heterostructures. Although the growth of epitaxial TI films on s-wave superconductors has been achieved, it remains an outstanding challenge for synthesizing atomically thin TI/superconductor heterostructures, which are critical for engineering the TSC phase. Here, we used MBE to grow Bi2Se3 films with the controlled thickness on monolayer NbSe2 and performed in-situ angle-resolved photoemission spectroscopy and ex-situ magneto-transport measurements on these Bi2Se3/monolayer NbSe2 heterostructures. We found that the emergence of Rashba-type bulk quantum well bands and spin-nondegenerate surface states coincides with a marked suppression of the in-plane upper critical magnetic field of the superconductivity in Bi2Se3/monolayer NbSe2 heterostructures. This is the signature of a crossover from Ising- to Rashba-type superconducting pairings, induced by altering Bi2Se3 film thickness. Our work opens a new route for exploring a robust TSC phase in TI/Ising superconductor heterostructures.

cond-mat.supr-con↗

arXiv2021

Electric control of a canted-antiferromagnetic Chern insulator

The interplay between band topology and magnetism can give rise to exotic states of matter. For example, magnetically doped topological insulators can realize a Chern insulator that exhibits quantized Hall resistance at zero magnetic field. While prior works have focused on ferromagnetic systems, little is known about band topology and its manipulation in antiferromagnets. Here, we report that MnBi$_2$Te$_4$ is a rare platform for realizing a canted-antiferromagnetic (cAFM) Chern insulator with electrical control. We show that the Chern insulator state with Chern number $C = 1$ appears as soon as the AFM to canted-AFM phase transition happens. The Chern insulator state is further confirmed by observing the unusual transition of the $C = 1$ state in the cAFM phase to the $C = 2$ orbital quantum Hall states in the magnetic field induced ferromagnetic phase. Near the cAFM-AFM phase boundary, we show that the Chern number can be toggled on and off by applying an electric field alone. We attribute this switching effect to the electrical field tuning of the exchange gap alignment between the top and bottom surfaces. Our work paves the way for future studies on topological cAFM spintronics and facilitates the development of proof-of-concept Chern insulator devices.

cond-mat.mes-hall↗

arXiv2020

Intertwined Topological and Magnetic Orders in Atomically Thin Chern Insulator MnBi2Te4

The interplay between band topology and magnetic order plays a key role in quantum states of matter. MnBi2Te4, a van der Waals magnet, has recently emerged as an exciting platform for exploring Chern insulator physics. Its layered antiferromagnetic order was predicted to enable even-odd layer-number dependent topological states, supported by promising edge transport measurements. Furthermore, it becomes a Chern insulator when all spins are aligned by an applied magnetic field. However, the evolution of the bulk electronic structure as the magnetic state is continuously tuned and its dependence on layer number remains unexplored. Here, employing multimodal probes, we establish one-to-one correspondence between bulk electronic structure, magnetic state, topological order, and layer thickness in atomically thin MnBi2Te4 devices. As the magnetic state is tuned through the canted magnetic phase, we observe a band crossing, i.e., the closing and reopening of the bulk bandgap, corresponding to the concurrent topological phase transition. Surprisingly, we find that the even- and odd-layer number devices exhibit a similar topological phase transition coupled to magnetic states, distinct from recent theoretical and experimental reports. Our findings shed new light on the interplay between band topology and magnetic order in this newly discovered topological magnet and validate the band crossing with concurrent measurements of topological invariant in a continuously tuned topological phase transition.

cond-mat.mes-hall↗

arXiv2020

Tunable correlation-driven symmetry breaking in twisted double bilayer graphene

A variety of correlated phases have recently emerged in select twisted van der Waals (vdW) heterostructures owing to their flat electronic dispersions. In particular, heterostructures of twisted double bilayer graphene (tDBG) manifest electric field-tunable correlated insulating (CI) states at all quarter fillings of the conduction band, accompanied by nearby states featuring signatures suggestive of superconductivity. Here, we report electrical transport measurements of tDBG in which we elucidate the fundamental role of spontaneous symmetry breaking within its correlated phase diagram. We observe abrupt resistivity drops upon lowering the temperature in the correlated metallic phases neighboring the CI states, along with associated nonlinear $I$-$V$ characteristics. Despite qualitative similarities to superconductivity, concomitant reversals in the sign of the Hall coefficient instead point to spontaneous symmetry breaking as the origin of the abrupt resistivity drops, while Joule heating appears to underlie the nonlinear transport. Our results suggest that similar mechanisms are likely relevant across a broader class of semiconducting flat band vdW heterostructures.

cond-mat.mes-hall↗

arXiv2017

Rasch Analysis of the Mathematics Self Concept Questionnaire

TIMSS & PIRLS International Study Center is a research center at Boston College that conducts a series of assessments in a number of countries to measure trends in mathematics and science achievement at the fourth and eighth grades. In general, TIMSS assessments include achievement tests as well as questionnaires for student, parent, teacher, school, and curricular. There are 63 participating countries and 14 benchmarking participants in TIMSS 2011, including 608,641 students, 49,429 teachers, 19,612 school principals, and the National Research Coordinators of each country. TIMSS data are valuable for researchers and analysts from all over the world, especially for those from participating countries, to conduct related studies to improve education.

stat.AP↗

arXiv2017

Effects of Home Resources and School Environment on Eighth-Grade Mathematics Achievement in Taiwan

Over the past decades, researchers have explored the relationship among home resources, school environment, and students' mathematics achievement in a large amount of studies. Many of them suggested that rich home resources for learning were related to higher average academic achievement. Some also suggested that the home background was closely associated with the learning environment, and therefore, influenced students' achievements. Thus, this study hypothesized that students who own more home resources would perform better than students who possess fewer resources and that schools that have more socioeconomically advantaged students, located in high-income neighborhoods, and possess more instructional resources would have better mathematics performance. The study focuses on eighth graders in Taiwan and explores the variance in mathematics achievement of students as a function of student and school level differences.

cs.CY↗