arXiv subjects

Binghai Yan

Publications and source records attributed to Binghai Yan.

At least 73 records · Page 4Linked to original sources

arXiv2025

Enantiomer-Selective Magnetoresistance in Chiral Gold Nanocrystals by Magnetic Control of Surface Potentials

Chiral nanomaterials offer intriguing possibilities for novel electronic and chemical applications. Here, we report the discovery of an enantiomer-selective magnetoresistance effect in chiral gold nanocrystals. Based on precise control of nanocrystal chiral morphology using amino acid-directed synthesis, we demonstrate that an external magnetic field can dramatically modulate resistance in an enantiomer-specific manner. For a given enantiomer, a magnetic field in one direction alters the resistance by dozens of times, while the opposite field direction leaves it unchanged. This asymmetric response reverses for the opposite enantiomer and are reproduced in both single nanocrystals by conduction atomic force microscopy and nanocrystal thin films in solid state devices. We attribute this phenomenon to a chirality-driven charge pumping effect, where the interplay between the chiral morphology and the magnetic field selectively modifies the surface potential. The magnitude and sign of the magnetoresistance can be further tuned by the surface chemistry of the nanocrystal, as demonstrated through sulfide treatment. Our findings reveal a new form of chirality-dependent magnetoresistance, distinct from previously known effects such as chirality-induced spin selectivity and electric magnetochiral anisotropy. The ability to remotely control surface potentials of chiral nanostructures using magnetic fields could enable novel approaches in catalysis, drug delivery, and nanoelectronics.

cond-mat.mes-hall↗

arXiv2025

Formation of Frustrated Charge Density Waves in Kagome Metal LuNb$_6$Sn$_6$

The charge density wave (CDW), a translational symmetry breaking electronic liquid, plays a pivotal role in correlated quantum materials, such as high-T$_c$ superconductors and topological semimetals. Recently, CDWs that possibly intertwine with superconductivity and magnetism are observed in various kagome metals. However, the nature of CDWs and the role of the Fermi surface (FS) topology in these materials remain an unresolved challenge. In this letter, we reveal the formation of CDWs in the newly discovered kagome metal LuNb$_6$Sn$_6$. We observe a precursor CDW correlation that features a "yield sign"-like hollow triangle diffuse scattering pattern and nearly complete softening of a flat optical phonon band near Q$_H$=(1/3, 1/3, 1/2). The scattering intensity of the precursor CDW displays divergent behavior as decreasing temperature until T$_{CDW}$=70 K, where a competing CDW at Q$_{CDW}$=(1/3, 1/3, 1/3) emerges. Using scanning tunneling microscopy/spectroscopy, we image the frustrated CDW patterns that show a short phase coherence length about ~20 nm in real space. Combined with first principles calculations, our observations support frustrated lattice interactions that are modulated by the FS topology. These results shed light on the interplay between FS and CDW in quantum materials with nearly degenerate structural deformation patterns.

cond-mat.str-el↗

arXiv2025

Designing Flat Bands, Localized and Itinerant States in TaS2 Trilayer Heterostructures

Stacking and twisting van der Waals materials provide a powerful tool to design quantum matter and engineer electron correlation. For instance, monolayers of 1T- and 1H-TaS2 are Mott insulating and metallic (also superconducting), respectively, and thus, the T/H bilayer systems have been extensively investigated in the context of heavy fermions and unconventional superconductivity, which are expected phases from localized spins (1T) coexisting with itinerant electrons (1H). However, recent studies revealed that significant charge transfer from the 1T to 1H layers removes the 1T Mottness and renders the above scenario elusive. In this work, we propose a T/T/H trilayer heterostructure by combining a T/T bilayer -- which is a band insulator with flat dispersion -- with a 1H layer. After charge redistribution, this trilayer heterostructure shows localized spins in the Mott flat band of the T/T bilayer and weak spin polarization in the metallic H layer. We argue that by varying the stacking configurations of the T/T bilayer in the T/T/H trilayer, a crossover from a doped Mott insulator to a Kondo insulator can be achieved. The T/T/H trilayer provides therefore a rich novel heterostructure platform to study strong correlation phenomena and unconventional superconductivity.

cond-mat.mtrl-sci↗

arXiv2025

Revealing the orbital origins of exotic electronic states with Ti substitution in kagome superconductor CsV3Sb5

The multiband kagome superconductor CsV3Sb5 exhibits complex orbital textures on the Fermi surface, making the orbital origins of its cascade of correlated electronic states and superconductivity a major scientific puzzle. Chemical doping of the kagome plane can simultaneously tune the exotic states and the Fermi-surface orbital texture, and thus offers a unique opportunity to correlate the given states with specific orbitals. In this Letter, by substituting V atoms with Ti in kagome superconductor CsV3Sb5, we reveal the orbital origin of a cascade of its correlated electronic states through the orbital-resolved quasiparticle interference (QPI). We analyze the QPI changes associated with different orbitals, aided by first-principles calculations. We have observed that the in-plane and out-of-plane vanadium 3d orbitals cooperate to form unidirectional coherent states in pristine CsV3Sb5, whereas the out-of-plane component disappears with doping-induced suppression of charge density wave and global electronic nematicity. In addition, the Sb pz orbital plays an important role in both the pseudo-gap and superconducting states in CsV3Sb5. Our findings offer new insights into multiorbital physics in quantum materials which are generally manifested with intriguing correlations between atomic orbitals and symmetry-encoded correlated electronic states.

cond-mat.supr-con↗

arXiv2024

Spectroscopic origin of giant anomalous Hall effect in an interwoven magnetic kagome metal

The discovery of a giant anomalous Hall effect (AHE) and its novel mechanism holds significant promise for advancing both fundamental research and practical applications. Magnetic kagome lattice materials are uniquely suited for studying the AHE due to their interplay between electronic structure, topology, and magnetism. However, the geometric frustration inherent in kagome lattices often limits the configuration and tunability of magnetic order. Here, we present a new design strategy for kagome-lattice materials with emergent magnetism, exemplified by the magnetic kagome metal TbTi$_3$Bi$_4$, which features interwoven magnetic Tb zigzag chains and non-magnetic Ti kagome bilayers. This material exhibits a record-high anomalous Hall conductivity (AHC) of 10$^5$ $Ω^{-1}$ cm$^{-1}$. Spectroscopy measurements reveal a large band folding gap observed via angle-resolved photoemission spectroscopy, coexisting spin-density-wave (SDW) order detected through spin-polarized scanning tunneling spectroscopy, and a spiral magnetic order with large magnetic moments identified by neutron diffraction. These findings highlight a strong electron-magnetic coupling between itinerant charges and ordered magnetic moments, offering a spectroscopic explanation for the giant AHC in TbTi$_3$Bi$_4$. This work establishes a pathway for innovative material design strategies, unlocking new possibilities for future exploration and applications in quantum and spintronic technologies.

cond-mat.str-el↗

arXiv2024

Magnetochiral Charge Pumping due to Charge Trapping and Skin Effect in Chirality-Induced Spin Selectivity

Chirality-induced spin selectivity (CISS) generates giant spin polarization in transport through chiral molecules, paving the way for novel spintronic devices and enantiomer separation. Unlike conventional transport, CISS magnetoresistance (MR) violates Onsager's reciprocal relation, exhibiting significant resistance changes when reversing electrode magnetization at zero bias. However, its underlying mechanism remains unresolved. In this work, we propose that CISS MR originates from charge trapping that modifies the electron tunneling barrier and circumvents Onsager's relation, distinct from previous spin polarization-based models. Charge trapping is governed by the non-Hermitian skin effect, where dissipation leads to exponential wavefunction localization at the ferromagnet-chiral molecule interface. Reversing magnetization or chirality alters the localization direction, changing the occupation of impurity/defect states in the molecule (i.e., charge trapping) -- a phenomenon we term magnetochiral charge pumping. Our theory explains why CISS MR can far exceed the ferromagnet spin polarization and why chiral molecules violate the reciprocal relation but chiral metals do not. Furthermore, it predicts exotic phenomena beyond the conventional CISS framework, including asymmetric MR induced by magnetic fields alone (without ferromagnetic electrodes), as confirmed by recent experiments. This work offers a deeper understanding of CISS and opens avenues for controlling electrostatic interactions in chemical and biological systems through the magnetochiral charge pumping.

cond-mat.mes-hall↗

arXiv2024

Visualizing incommensurate inter-valley coherent states in rhombohedral trilayer graphene

ABC-stacked rhombohedral graphene multilayers exhibit a wide variety of electronic ground states characterized by broken isospin symmetry and superconductivity. Recently, indirect evidence of inter-valley coherent (IVC) order has been reported in rhombohedral trilayer graphene (RTG), with possible implications for the origin of superconductivity. Here, we report the direct visualization of IVC order in RTG using scanning tunneling microscopy and spectroscopy. Tuning the chemical potential through the Van Hove singularity near the edge of the valence band, we observe a cascade of phase transitions associated with the formation of half- and quarter-metal states. IVC phases, distinguished by an enlarged real space unit cell, are directly imaged near both the high- and low-density boundaries of the half-metal phase. At high hole density, we precisely reconstruct the IVC band structure through quasiparticle interference. Intriguingly, the charge density modulations reveal a C3-symmetric incommensurate IVC order that agrees with the recent prediction of an IVC-crystal phase. Our findings demonstrate that IVC phases are a widespread symmetry-broken ground state within graphene systems.

cond-mat.mes-hall↗

arXiv2024

Disordered charge density waves in the kagome metal FeGe

The discovery of a charge density wave (CDW) in the antiferromagnetic kagome metal FeGe has prompted interest in the interplay between kagome physics, CDW, and magnetism. However, a crucial aspect for understanding these emergent phenomena-the precise CDW structure-remains ambiguous. Recent studies have assumed uniformly distributed Ge dimers, but this assumption is problematic. The predicted band structure based on this model exhibits an abrupt disappearance of a Ge-$p$ band in the Fermi surface, contradicting experimental observations from angle-resolved photoemission spectroscopy (ARPES). In this study, we propose that a CDW phase with disordered Ge dimers can reconcile theoretical predictions with ARPES results. This model reproduces the observed CDW gaps while preserving the Ge-$p$ band. Depending on experimental conditions, Ge dimers can be randomly distributed or exhibit phase separation from pristine regions. Our findings reveal the crucial role of Ge dimer disorder in the FeGe CDW and suggest potential implications of this disorder for other properties, such as magnetism and transport, in this system.

cond-mat.mtrl-sci↗

arXiv2024

Persistent flat band splitting and strong selective band renormalization in a kagome magnet thin film

Magnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X=Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While partially filling a kagome flat band is predicted to give rise to a Stoner-type ferromagnetism, experimental visualization of the magnetic splitting across the ordering temperature has not been reported for any of these systems due to the high ordering temperatures, hence leaving the nature of magnetism in kagome magnets an open question. Here, we probe the electronic structure with angle-resolved photoemission spectroscopy in a kagome magnet thin film FeSn synthesized using molecular beam epitaxy. We identify the exchange-split kagome flat bands, whose splitting persists above the magnetic ordering temperature, indicative of a local moment picture. Such local moments in the presence of the topological flat band are consistent with the compact molecular orbitals predicted in theory. We further observe a large spin-orbital selective band renormalization in the Fe d_xy+d_(x^2-y^2 ) spin majority channel reminiscent of the orbital selective correlation effects in the iron-based superconductors. Our discovery of the coexistence of local moments with topological flat bands in a kagome system echoes similar findings in magic-angle twisted bilayer graphene, and provides a basis for theoretical effort towards modeling correlation effects in magnetic flat band systems.

cond-mat.mes-hall↗

arXiv2024

Pomeranchuk Instability Induced by an Emergent Higher-Order van Hove Singularity on the Distorted Kagome Surface of Co$_3$Sn$_2$S$_2$

Materials hosting flat bands at the vicinity of the Fermi level promote exotic symmetry broken states. Common to many of these are van Hove singularities at saddle points of the dispersion or even higher-order van Hove singularities where the dispersion is flattened further. The band structure of kagome metals hosts both a flat band and two regular saddle points flanking a Dirac node. We investigate the kagome ferromagnetic metal Co$_3$Sn$_2$S$_2$ using scanning tunneling spectroscopy. We identify a new mechanism by which a triangular distortion on its kagome Co$_3$Sn surface termination considerably flattens the saddle point dispersion, and induces an isolated higher-order van Hove singularity (HOvHS) with algebraically divergent density of states pinned to the Fermi energy. The distortion-induced HOvHS precipitates a Pomeranchuk instability of the Fermi surface, resulting in the formation of a series of nematic electronic states. We visualize the nematic order across an energy shell of about 100 meV in both real-, reciprocal-, and momentum-spaces, as a cascade of wavefunction distributions which spontaneously break the remaining rotational symmetry of the underlying distorted kagome lattice, without generating any additional translational symmetry breaking. It signifies the spontaneous removal of a subset of saddle points from the Fermi energy to lower energies. By tracking the electronic wavefunction structure across the deformed Fermi surface we further identify a charge pumping-like evolution of the wavefunction center of mass. The mechanism we find for the generation of higher-order saddle points under a kagome distortion may be common to other kagome materials, and potentially other lattice structures, suggesting a generic new avenue for inducing unconventional electronic instabilities towards exotic states of matter.

cond-mat.str-el↗

arXiv2024

Enhanced Magnetization by Defect-Assisted Exciton Recombination in Atomically Thin CrCl$_3$

Two dimensional (2D) semiconductors present unique opportunities to intertwine optical and magnetic functionalities and to tune these performances through defects and dopants. Here, we integrate exciton pumping into a quantum sensing protocol on nitrogen-vacancy centers in diamond to image the optically-induced transient stray fields in few-layer, antiferromagnetic CrCl$_3$. We discover that exciton recombination enhances the in-plane magnetization of the CrCl$_3$ layers, with a predominant effect in the surface monolayers. Concomitantly, time-resolved photoluminescence measurements reveal that nonradiative exciton recombination intensifies in atomically thin CrCl$_3$ with tightly localized, nearly dipole-forbidden excitons and amplified surface-to-volume ratio. Supported by experiments under controlled surface exposure and density functional theory calculations, we interpret the magnetically enhanced state to result from a defect-assisted Auger recombination that optically activates electron transfer between water vapor related surface impurities and the spin-polarized conduction band. Our work validates defect engineering as a route to enhance intrinsic magnetism in single magnetic layers and opens a novel experimental platform for studying optically-induced, transient magnetism in condensed matter systems.

cond-mat.mtrl-sci↗

arXiv2024

Emergent superconductivity and pair density wave at antiphase boundaries of charge density wave order in kagome metals

Central to the layered kagome lattice superconductors AV3Sb5 (A = K, Cs, Rb) is a cascade of novel quantum states triggered by an unconventional charge density wave (CDW) order. The three-dimensional (3D) order involves a 2x2x2 phase coherent stacking of 2x2 charge density modulations in the kagome plane at low temperatures, exhibiting a CDW energy gap and evidence for time-reversal symmetry breaking. Here we report the discovery of emergent superconductivity and primary pair density wave (PDW) at the antiphase boundaries and stacking faults of bulk CDW order. We find that the π-phase shift dislocations can naturally appear on the surface as the Cs atoms form 2x2 superstructures that are out of phase with the bulk CDW. An incipient narrow band of surface states inside bulk CDW gap emerge close to the Fermi level where a particle-hole symmetric energy gap develops. We demonstrate that the energy gap originates from a novel quasi-2D kagome superconducting state (Tc ~ 5.4 K) intertwined with bulk CDW order, exhibiting an unprecedented vortex core spectrum and spatial modulations of the superconducting gap consistent with a 4x4 PDW. Intriguingly, the 2D kagome superconductivity is shown to be tunable on and off by atomically manipulating the Cs atoms on the surface. Our findings provide fresh new insights for understanding the interplay between the unconventional CDW and superconductivity in kagome metals and a pathway for atomic manipulation and topological defects engineering of quantum many-body states in correlated materials.

cond-mat.supr-con↗

arXiv2024

Revealing the Berry phase under the tunneling barrier

In quantum mechanics, a quantum wavepacket may acquire a geometrical phase as it evolves along a cyclic trajectory in parameter space. In condensed matter systems, the Berry phase plays a crucial role in fundamental phenomena such as the Hall effect, orbital magnetism, and polarization. Resolving the quantum nature of these processes commonly requires sensitive quantum techniques, as tunneling, being the dominant mechanism in STM microscopy and tunneling transport devices. In this study, we integrate these two phenomena - geometrical phases and tunneling - and observe a complex-valued Berry phase via strong field light matter interactions in condensed matter systems. By manipulating the tunneling barrier, with attoseconds precision, we measure the imaginary Berry phase accumulated as the electron tunnels during a fraction of the optical cycle. Our work opens new theoretical and experimental directions in geometrical phases physics and their realization in condensed matter systems, expanding solid state strong field light metrology to study topological quantum phenomena.

cond-mat.mes-hall↗

arXiv2024

Hybrid-order topology in unconventional magnets of Eu-based Zintl compounds with surface-dependent quantum geometry

The exploration of magnetic topological insulators is instrumental in exploring axion electrodynamics and intriguing transport phenomena, such as the quantum anomalous Hall effect. Here, we report that a family of magnetic compounds Eu$_{2n+1}$In$_{2}$(As,Sb)$_{2n+2}$ ($n=0,1,2$) exhibit both gapless Dirac surface states and chiral hinge modes. Such a hybrid-order topology hatches surface-dependent quantum geometry. By mapping the responses into real space, we demonstrate the existence of chiral hinge modes along the $c$ direction, which originate from the half-quantized anomalous Hall effect on two gapped $ac$/$bc$ facets due to Berry curvature, while the unpinned Dirac surface states on the gapless $ab$ facet generate an intrinsic nonlinear anomalous Hall effect due to the quantum metric. When Eu$_{3}$In$_{2}$As$_{4}$ is polarized to the ferromagnetic phase by an external magnetic field, it becomes an ideal Weyl semimetal with a single pair of type-I Weyl points and no extra Fermi pocket. Our work predicts rich topological states sensitive to magnetic structures, quantum geometry-induced transport and topological superconductivity if proximitized with a superconductor.

cond-mat.mtrl-sci↗

arXiv2024

Distinguishing Surface and Bulk Electromagnetism via Their Dynamics in an Intrinsic Magnetic Topological Insulator

The indirect exchange interaction between local magnetic moments via surface electrons has been long predicted to bolster the surface ferromagnetism in magnetic topological insulators (MTIs), which facilitates the quantum anomalous Hall effect. This unconventional effect is critical to determining the operating temperatures of future topotronic devices. However, the experimental confirmation of this mechanism remains elusive, especially in intrinsic MTIs. Here we combine time-resolved photoemission spectroscopy with time-resolved magneto-optical Kerr effect measurements to elucidate the unique electromagnetism at the surface of an intrinsic MTI MnBi2Te4. Theoretical modeling based on 2D Ruderman-Kittel-Kasuya-Yosida interactions captures the initial quenching of a surface-rooted exchange gap within a factor of two but over-estimates the bulk demagnetization by one order of magnitude. This mechanism directly explains the sizable gap in the quasi-2D electronic state and the nonzero residual magnetization in even-layer MnBi2Te4. Furthermore, it leads to efficient light-induced demagnetization comparable to state-of-the-art magnetophotonic crystals, promising an effective manipulation of magnetism and topological orders for future topotronics.

cond-mat.str-el↗

arXiv2024

Even-integer Quantum Hall Effect in an Oxide Caused by Hidden Rashba Effect

In the presence of high magnetic field, quantum Hall systems usually host both even- and odd-integer quantized states because of lifted band degeneracies. Selective control of these quantized states is challenging but essential to understand the exotic ground states and manipulate the spin textures. Here, we study the quantum Hall effect in Bi2O2Se thin films. In magnetic fields as high as 50 T, we observe only even-integer quantum Hall states, but no sign of odd-integer states. However, when reducing the thickness of the epitaxial Bi2O2Se film to one unit cell, we observe both odd- and even-integer states in this Janus (asymmetric) film grown on SrTiO3. By means of a Rashba bilayer model based on ab initio band structures of Bi2O2Se thin films, we can ascribe the absence of odd-integer states in thicker films to the hidden Rasbha effect, where the local inversion symmetry breaking in two sectors of the [Bi2O2]2+ layer yields opposite Rashba spin polarizations, which compensate with each other. In the one unit cell Bi2O2Se film grown on SrTiO3, the asymmetry introduced by top surface and bottom interface induces a net polar field. The resulting global Rashba effect lifts the band degeneracies present in the symmetric case of thicker films.

cond-mat.mes-hall↗

arXiv2024

Topotaxial Mutual-Exchange Growth of Magnetic Zintl Eu$_3$In$_2$As$_4$ Nanowires with Axion Insulator Classification

Nanomaterials bring to expression unique electronic properties that promote advanced functionality and technologies. Albeit, nanoscale growth presents paramount challenges for synthesis limiting the diversity in structures and compositions. Here, we demonstrate solid-state topotactic exchange that converts Wurtzite InAs nanowires into Zintl phase Eu$_3$In$_2$As$_4$ nanowires. In situ evaporation of Eu and As over InAs nanowire cores in molecular beam epitaxy results in mutual exchange of Eu from the shell and In from the core. A continuous Eu$_3$In$_2$As$_4$ shell thereby grows that gradually consumes the InAs core and converts it into a single phase Eu$_3$In$_2$As$_4$ nanowire. Topotaxy, which facilitates the mutual exchange, is supported by the substructure of the As matrix which is similar across the Wurtzite InAs and Zintl Eu$_3$In$_2$As$_4$. We provide initial evidence of an antiferromagnetic transition at T$_N$ $\sim$ 6.5 K in the Zintl phase Eu$_3$In$_2$As$_4$ nanowires. Ab initio calculation confirms the antiferromagnetic state and classifies Eu$_3$In$_2$As$_4$ as a $C_2 T$ axion insulator hosting both chiral hinge modes and unpinned Dirac surface states. The topotactic mutual-exchange growth of Zintl Eu$_3$In$_2$As$_4$ nanowires thus enables the exploration of intricate magneto-topological states of nanomaterials. Moreover, it may open the path for topotactic mutual-exchange synthesis of nanowires made of other exotic compounds.

cond-mat.mtrl-sci↗

arXiv2024

Massive 1D Dirac Line, Solitons and Reversible Manipulation on the Surface of a Prototype Obstructed Atomic Insulator, Silicon

Topologically trivial insulators can be classified into atomic insulators (AIs) and obstructed atomic insulators (OAIs) depending on whether the Wannier charge centers are localized or not at spatial positions occupied by atoms. An OAI can possess unusual properties such as surface states along certain crystalline surfaces, which advantageously appear in materials with much larger bulk energy gap than topological insulators, making them more attractive for potential applications. In this work, we show that a well-known crystal, silicon (Si) is a model OAI, which naturally explains some of Si's unusual properties such as its famous (111) surface states. On this surface, using angle resolved photoemission spectroscopy (ARPES), we reveal sharp quasi-1D massive Dirac line dispersions; we also observe, using scanning tunneling microscopy/spectroscopy (STM/STS), topological solitons at the interface of the two atomic chains. Remarkably, we show that the different chain domains can be reversibly switched at the nanometer scale, suggesting the application potential in ultra-high density storage devices.

cond-mat.mes-hall↗