arXiv subjects

Jiaqiang Yan

Publications and source records attributed to Jiaqiang Yan.

At least 109 records · Page 6Linked to original sources

arXiv2021

Topological surface currents accessed through reversible hydrogenation of the three-dimensional bulk

Hydrogen, the smallest and most abundant element in nature, can be efficiently incorporated within a solid and drastically modify its electronic state - it has been known to induce novel magnetoelectric effects in complex perovskites and modulate insulator-to-metal transition in a correlated Mott oxide. Here we demonstrate that hydrogenation resolves an outstanding challenge in chalcogenide classes of three-dimensional (3D) topological insulators and magnets - the control of intrinsic bulk conduction that denies access to quantum surface transport. With electrons donated by a reversible binding of H+ ions to Te(Se) chalcogens, carrier densities are easily changed by over 10^20 cm^-3, allowing tuning the Fermi level into the bulk bandgap to enter surface/edge current channels. The hydrogen-tuned topological materials are stable at room temperature and tunable disregarding bulk size, opening a breadth of platforms for harnessing emergent topological states.

cond-mat.mtrl-sci↗

arXiv2021

Direct evidence of ferromagnetism in MnSb2Te4

We report the magnetic imaging of ferromagnetic domains in the van der Waals single crystal MnSb2Te4 from two different sources using cryogenic magnetic force microscopy. The magnetic field dependence of the domains reveals very weak pinning of domain walls in MnSb2Te4, resulting in a negligibly small magnetic hysteresis loop. The temperature dependence of the domain contrast reveals a mean field like behavior, in good agreement with that of bulk magnetization measurements.

cond-mat.mtrl-sci↗

arXiv2021

Intrinsic Donor-Bound Excitons in Ultraclean Monolayer Semiconductors

The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe2. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (> 6 $μ$s) and polarization lifetimes (> 100 ns). Resonant excitation of the free inter- and intra-valley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellites photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors.

cond-mat.mes-hall↗

arXiv2021

Thermal and magnetoelastic properties of α-RuCl3 in the field-induced low temperature states

We discuss the implications that new magnetocaloric, thermal expansion and magnetostriction data in $α$-RuCl$_{3}$ single crystals have on its temperature-field phase diagram and uncover the magnetic-field dependence of an apparent energy gap structure $Δ(H)$ that evolves when the low temperature antiferromagnetic order is suppressed. We show that, depending on how the thermal expansion data is modeled, $Δ(H)$ can show a cubic field dependence and remain finite at zero field, consistent with the pure Kitaev model hosting itinerant Majorana fermions and localized $\mathbb{Z}_{2}$ fluxes. Our magnetocaloric effect data provides, below $1\,\mathrm{K}$, unambiguous evidence for dissipative phenomena at $H_{\mathrm{c}}$, smoking gun for a first order phase transition. Our results, on the other hand, show little support for a phase transition from a QSL to a polarized paramagnetic state above $H_{\mathrm{c}}$.

cond-mat.str-el↗

arXiv2020

Bayesian learning of adatom interactions from atomically-resolved imaging data

Atomic structures and adatom geometries of surfaces encode information about the thermodynamics and kinetics of the processes that lead to their formation, and which can be captured by a generative physical model. Here we develop a workflow based on a machine learning-based analysis of scanning tunneling microscopy images to reconstruct the atomic and adatom positions, and a Bayesian optimization procedure to minimize statistical distance between the chosen physical models and experimental observations. We optimize the parameters of a 2- and 3-parameter Ising model describing surface ordering and use the derived generative model to make predictions across the parameter space. For concentration dependence, we compare the predicted morphologies at different adatom concentrations with the dissimilar regions on the sample surfaces that serendipitously had different adatom concentrations. The proposed workflow is universal and can be used to reconstruct the thermodynamic models and associated uncertainties from the experimental observations of materials microstructures. The code used in the manuscript is available at https://github.com/saimani5/Adatom_interactions.

cond-mat.mtrl-sci↗

arXiv2020

Field-free deterministic switching of a perpendicularly polarized magnet using unconventional spin-orbit torques in WTe2

Spin-orbit torque (SOT) driven deterministic control of the magnetization state of a magnet with perpendicular magnetic anisotropy (PMA) is key to next generation spintronic applications including non-volatile, ultrafast, and energy efficient data storage devices. But, field-free deterministic switching of perpendicular magnetization remains a challenge because it requires an out-of-plane anti-damping torque, which is not allowed in conventional spin source materials such as heavy metals (HM) 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. Here, we report the first experimental realization of field-free deterministic magnetic switching of a perpendicularly polarized van der Waals (vdW) magnet employing an out-of-plane anti-damping SOT generated in layered WTe2 which is a low-crystal symmetry quantum material. The numerical simulations confirm that out-of-plane antidamping torque in WTe2 is responsible for the observed magnetization switching in the perpendicular direction.

cond-mat.mes-hall↗

arXiv2020

Induced anomalous Hall effect of massive Dirac fermions in ZrTe5 and HfTe5 thin flakes

Researches on anomalous Hall effect (AHE) have been lasting for a century to make clear the underlying physical mechanism. Generally, the AHE appears in magnetic materials, in which extrinsic process related to scattering effects and intrinsic contribution connected with Berry curvature are crucial. Recently, AHE has been counterintuitively observed in non-magnetic topological materials and attributed to the existence of Weyl points. However, the Weyl point scenario would lead to unsaturated AHE even in large magnetic fields and contradicts the saturation of AHE in several tesla (T) in experiments. In this work, we investigate the Hall effect of ZrTe5 and HfTe5 thin flakes in static ultrahigh magnetic fields up to 33 T. We find the AHE saturates to 55 (70) Ohm^-1*cm^-1 for ZrTe5 (HfTe5) thin flakes above ~ 10 T. Combining detailed magnetotransport experiments and Berry curvature calculations, we clarify that the splitting of massive Dirac bands without Weyl points can be responsible for AHE in non-magnetic topological materials ZrTe5 and HfTe5 thin flakes. This model can identify our thin flake samples to be weak topological insulators and serve as a new tool to probe the band structure topology in topological materials.

cond-mat.mtrl-sci↗

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

Tunable discrete scale invariance in transition-metal pentatelluride flakes

Log-periodic quantum oscillations discovered in transition-metal pentatelluride give a clear demonstration of discrete scale invariance (DSI) in solid-state materials. The peculiar phenomenon is convincingly interpreted as the presence of two-body quasi-bound states in a Coulomb potential. However, the modifications of the Coulomb interactions in many-body systems having a Dirac-like spectrum are not fully understood. Here, we report the observation of tunable log-periodic oscillations and DSI in ZrTe5 and HfTe5 flakes. By reducing the flakes thickness, the characteristic scale factor is tuned to a much smaller value due to the reduction of the vacuum polarization effect. The decreasing of the scale factor demonstrates the many-body effect on the DSI, which has rarely been discussed hitherto. Furthermore, the cut-offs of oscillations are quantitatively explained by considering the Thomas-Fermi screening effect. Our work clarifies the many-body effect on DSI and paves a way to tune the DSI in quantum materials.

cond-mat.mtrl-sci↗

arXiv2020

Quantum Oscillations in the Field-induced Ferromagnetic State of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$

The intrinsic antiferromagnetic topological insulator MnBi$_{2}$Te$_{4}$ undergoes a metamagnetic transition in a c-axis magnetic field. It has been predicted that ferromagnetic MnBi$_{2}$Te$_{4}$ is an ideal Weyl semimetal with a single pair of Weyl nodes. Here we report measurements of quantum oscillations detected in the field-induced ferromagnetic phase of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$, where Sb substitution tunes the majority carriers from electrons to holes. Single frequency Shubnikov-de Haas oscillations were observed in a wide range of Sb concentrations (0.54 $\leq$ x $\leq$ 1.21). The evolution of the oscillation frequency and the effective mass shows reasonable agreement with the Weyl semimetal band-structure of ferromagnetic MnBi$_{2}$Te$_{4}$ predicted by density functional calculations. Intriguingly, the quantum oscillation frequency shows a strong temperature dependence, indicating that the electronic structure sensitively depends on magnetism.

cond-mat.mtrl-sci↗

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

Native Defects in Antiferromagnetic Topological Insulator MnBi$_2$Te$_4$

Using scanning tunneling microscopy and spectroscopy, we visualized the native defects in antiferromagnetic topological insulator $\mathrm{MnBi_2Te_4}$. Two native defects $\mathrm{Mn_{Bi}}$ and $\mathrm{Bi_{Te}}$ antisites can be well resolved in the topographic images. $\mathrm{Mn_{Bi}}$ tend to suppress the density of states at conduction band edge. Spectroscopy imaging reveals a localized peak-like local density of state at $\sim80$~meV below the Fermi energy. A careful inspection of topographic and spectroscopic images, combined with density functional theory calculation, suggests this results from $\mathrm{Bi_{Mn}}$ antisites at Mn sites. The random distribution of $\mathrm{Mn_{Bi}}$ and $\mathrm{Bi_{Mn}}$ antisites results in spatial fluctuation of local density of states near the Fermi level in $\mathrm{MnBi_2Te_4}$.

cond-mat.mtrl-sci↗

arXiv2020

Surface Superconductivity in the type II Weyl Semimetal TaIrTe4

The search for unconventional superconductivity in Weyl semimetal materials is currently an exciting pursuit, since such superconducting phases could potentially be topologically nontrivial and host exotic Majorana modes. The layered material TaIrTe4 is a newly predicted time-reversal invariant type II Weyl semimetal with minimum number of Weyl points. Here, we report the discovery of surface superconductivity in Weyl semimetal TaIrTe4. Our scanning tunneling microscopy/spectroscopy (STM/S) visualizes Fermi arc surface states of TaIrTe4 that are consistent with the previous angle-resolved photoemission spectroscopy (ARPES) results. By a systematic study based on STS at ultralow temperature, we observe uniform superconducting gaps on the sample surface. The superconductivity is further confirmed by electrical transport measurements at ultralow temperature, with an onset transition temperature (Tc) up to 1.54 K being observed. The normalized upper critical field h*(T/Tc) behavior and the stability of the superconductivity against the ferromagnet indicate that the discovered superconductivity is unconventional with the p-wave pairing. The systematic STS, thickness and angular dependent transport measurements reveal that the detected superconductivity is quasi-one-dimensional (quasi-1D) and occurs in the surface states. The discovery of the surface superconductivity in TaIrTe4 provides a new novel platform to explore topological superconductivity and Majorana modes.

cond-mat.supr-con↗

arXiv2020

Unconventional Hall Effect induced by Berry Curvature

Berry phase and Berry curvature play a key role in the development of topology in physics and do contribute to the transport properties in solid state systems. In this paper, we report the finding of novel nonzero Hall effect in topological material ZrTe5 flakes when in-plane magnetic field is parallel and perpendicular to the current. Surprisingly, both symmetric and antisymmetric components with respect to magnetic field are detected in the in-plane Hall resistivity. Further theoretical analysis suggests that the magnetotransport properties originate from the anomalous velocity induced by Berry curvature in a tilted Weyl semimetal. Our work not only enriches the Hall family but also provides new insights into the Berry phase effect in topological materials.

cond-mat.mes-hall↗

arXiv2020

Spin dynamics and a nearly continuous magnetic phase transition in an entropy-stabilized oxide antiferromagnet

The magnetic order and the spin dynamics in the antiferromagnetic entropy-stabilized oxide (Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$)O (MgO-ESO) have been studied using muon spin relaxation ($μ$SR) and inelastic neutron scattering. We find that antiferromagnetic order develops gradually in the sample volume as it is cooled below 140 K, becoming fully ordered around 100 K. The spin dynamics show a critical slowing down in the vicinity of the transition, and the magnetic order parameter grows continuously in the ordered state. These results indicate that the antiferromagnetic transition is continuous but proceeds with a Gaussian distribution of ordering temperatures. The magnetic contribution to the specific heat determined from inelastic neutron scattering likewise shows a broad feature centered around 120 K. High-resolution inelastic neutron scattering further reveals an initially gapped spectrum at low temperature which sees an increase in a quasielastic contribution upon heating until the ordering temperature.

cond-mat.mtrl-sci↗

arXiv2020

Tuning Fermi Levels in Intrinsic Antiferromagnetic Topological Insulators MnBi2Te4 and MnBi4Te7 by Defect Engineering and Chemical Doping

MnBi2Te4 and MnBi4Te7 are intrinsic antiferromagnetic topological insulators, offering a promising materials platform for realizing exotic topological quantum states. However, high densities of intrinsic defects in these materials not only cause bulk metallic conductivity, preventing the measurement of quantum transport in surface states, but may also affect magnetism and topological properties. In this paper, we show by density functional theory calculations that the strain induced by the internal heterostructure promotes the formation of large-size-mismatched antisite defect BiMn in MnBi2Te4; such strain is further enhanced in MnBi4Te7, giving rise to even higher BiMn density. The abundance of intrinsic BiMn donors results in degenerate n-type conductivity under the Te-poor growth condition. Our calculations suggest that growths in a Te-rich condition can lower the Fermi level, which is supported by our transport measurements. We further show that the internal strain can also enable efficient doping by large-size-mismatched substitutional NaMn acceptors, which can compensate BiMn donors and lower the Fermi level. Na doping may pin the Fermi level inside the bulk band gap even at the Te-poor limit in MnBi2Te4. Furthermore, facile defect formation in MnSb2Te4 and its implication in Sb doping in MnBi2Te4 as well as the defect segregation in MnBi4Te7 are discussed. The defect engineering and doping strategies proposed in this paper will stimulate further studies for improving synthesis and for manipulating magnetic and topological properties in MnBi2Te4, MnBi4Te7, and related compounds.

cond-mat.mtrl-sci↗

arXiv2020

Robust A-type order and spin-flop transition on the surface of the antiferromagnetic topological insulator MnBi$_2$Te$_4$

Here we present microscopic evidence of the persistence of uniaxial A-type antiferromagnetic order to the surface layers of MnBi$_2$Te$_4$ single crystals using magnetic force microscopy. Our results reveal termination-dependent magnetic contrast across both surface step edges and domain walls, which can be screened by thin layers of soft magnetism. The robust surface A-type order is further corroborated by the observation of termination-dependent surface spin-flop transitions, which have been theoretically proposed decades ago. Our results not only provide key ingredients for understanding the electronic properties of the antiferromagnetic topological insulator MnBi$_2$Te$_4$, but also open a new paradigm for exploring intrinsic surface metamagnetic transitions in natural antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2020

One-Dimensional Edge Transport in Few-Layer WTe$_2$

WTe$_2$ is a layered transitional-metal dichalcogenide (TMD) with a number of intriguing topological properties. Recently, WTe$_2$ has been predicted to be a higher-order topological insulator (HOTI) with topologically protected hinge states along the edges. The gapless nature of WTe$_2$ complicates the observation of one-dimensional (1D) topological states in transport due to their small contribution relative to the bulk. Here, we study the behavior of the Josephson effect in magnetic field to distinguish edge from bulk transport. The Josephson effect in few-layer WTe$_2$ reveals 1D states residing on the edges and steps. Moreover, our data demonstrates a combination of Josephson transport properties observed solely in another HOTI - bismuth, including Josephson transport over micrometer distances, extreme robustness in a magnetic field, and nonsinusoidal current-phase relation (CPR). Our observations strongly suggest the topological origin of the 1D states and that few-layer WTe$_2$ is a HOTI.

cond-mat.mes-hall↗