arXiv subjects

Zhenzhong Yang

Publications and source records attributed to Zhenzhong Yang.

16 recordsLinked to original sources

arXiv2026

Spin Fluctuations-induced Unconventional Transverse Spin Current in Spin Degenerate Antiferromagnet

Modern magnetic memory technology requires unconventional transverse spin current to achieve deterministic switching of perpendicular magnetization. Spin current in antiferromagnets (AFMs) has been long thought to be trivial as nonmagnets. Recently, a class of spin-splitting AFMs has been shown to be able to generate unconventional spin current for spin-orbit torque (SOT) applications. However, such AFMs requires specific symmetry breaking, and have been largely restricted by the limited material candidates. Here, we reveal universal spin fluctuation-induced unconventional transverse spin current in conventional spin degenerate AFMs at finite temperature by taking collinear (AFM) L10-MnPt as an example. The field-free switching of perpendicular magnetization was achieved via out-of-plane anti-damping-like SOT, leveraging the spin current with both y- and z-spin polarizations in heterostructures with MnPt functioning as spin current source. Based on symmetry analyses and experimental characterizations of current-induced spin torques, we find that the spin current generated by L10-MnPt exhibits anisotropy that varies with the current direction, enforced by the low symmetry magnetic point group when its Néel vector is along [110]. From a fundamental perspective, it would be intriguing to uncover a mechanism underlying the emergence of unconventional spin currents in spin degenerate antiferromagnets, which is highly desirable for spintronic applications.

physics.app-ph↗

arXiv2026

Ising superconductivity and anomalous metallic states in a bulk crystal with artificial unidirectional stacking layers

The two-dimensional (2D) limit in macroscopic bulk crystals provides a powerful platform for exploring exotic quantum phases. Here, we report the synthesis of a Sr0.75ClNbS2 superconductor that achieves unidirectional, parallel AA stacking-a configuration never before realized in a bulk crystal. Unlike conventional intercalation, which merely expands the interlayer spacing, our approach employs a planar Sr-Cl network to enforce a complete stacking reorganization, driving all NbS2 layers from the native antiparallel AB stacking into a unidirectional, parallel AA arrangement. This stacking switch globally breaks inversion symmetry, transforming centrosymmetric 2H-NbS2 into a noncentrosymmetric bulk crystal with D3h point group symmetry. Crucially, this structural design reproduces, in three dimensions, the electronic environment of an isolated monolayer, thereby preventing cancellation of the local Ising fields. As a result, strong Ising spin-orbit coupling and spin-split bands persist throughout the bulk. Transport measurements reveal extreme superconducting anisotropy (γ ~ 77), an in-plane upper critical field (~ 10.65 T) that far exceeds the Pauli paramagnetic limit, and clean-limit superconductivity indicative of high crystalline quality. Moreover, magnetotransport uncovers a novel magnetic-field-induced anomalous metallic state characterized by finite dissipation yet a vanishing Hall response. Direct band-structure measurements corroborate the layer-decoupled, quasi-2D electronic nature of the system. This work establishes stacking-geometry engineering as a powerful strategy to artificially enforce a globally noncentrosymmetric, quasi-2D superconducting state in bulk crystals, paving the way for designing quantum materials with tunable crystalline symmetry and electronic band topology.

cond-mat.supr-con↗

arXiv2026

Breaking the Trade-off: Bulk 2D Ising Superconductivity with High Tc and Giant Interlayer Spacing via a Unique Chain Intercalation in (BaS)1/3TaS2

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising platforms for low dimensional superconductivity. However, in conventional intercalated systems, achieving a high superconducting transition temperature (Tc) often comes at the expense of reduced interlayer spacing and weakened 2D character. Here, we overcome this long-standing compromise through a unique chain-like intercalation strategy. We report the synthesis and properties of a new polymorph, (BaS)1/3TaS2, in which a distinctive Ba-S-S-Ba chain structure is inserted between TaS2 bilayers. This unique configuration breaks the bulk c axis mirror symmetry while achieving exceptional interlayer decoupling, with an inter-bilayer spacing of 12.75 Å-more than three times that of pristine 2H-TaS2. By suppressing interlayer electronic coupling, this structural evolution allows local inversion symmetry breaking within individual TaS2 layers to dominate. This prevents compensation of the Ising spin-orbit fields typical of centrosymmetric bulk phases, enabling robust 2D Ising superconductivity. Remarkably, the compound exhibits an enhanced Tc without sacrificing its large interlayer spacing, thereby breaking the conventional trade-off between large spacing/high anisotropy and high Tc. Comprehensive transport, magnetic, and thermodynamic measurements confirm its robust superconducting state. Our work establishes a versatile intercalation framework for designing bulk-like 2D Ising superconductors, providing a new route to reconcile competing material demands and expanding the scope of Ising superconductivity research.

cond-mat.supr-con↗

arXiv2025

Sliding two-dimensional superconductivity and charge-density-wave state in a bulk crystal

Superconductivity in the two-dimensional (2D) limit is a fertile ground for exotic quantum phenomena-many of which remain elusive in their 3D counterparts. While studies of 2D superconductivity have predominantly focused on mono- or few-layer systems, we demonstrate an alternative route-interlayer sliding in bulk crystals. Through a precisely controlled growth strategy, we engineer interlayer sliding in bulk 3R-NbSe2, deliberately disrupting [001] mirror symmetry and drastically suppressing interlayer coupling. Remarkably, this structural manipulation stabilizes Ising-type superconductivity coexisting with an unconventional charge-density-wave (CDW) state akin to that of monolayer 2H-NbSe2. The sliding phase exhibits a pronounced suppression of the upper critical field at low temperatures, revealing a delicate competition between Ising and Rashba spin-orbit coupling (SOC) in the globally noncentrosymmetric lattice. Intriguingly, the superconducting state displays two-fold symmetry, a signature that may arise from asymmetric SOC or a multi-component pairing order parameter. Our work establishes interlayer sliding as a symmetry-breaking tool to promote 2D superconductivity in bulk materials-without resorting to extrinsic intercalation or doping. More broadly, this approach sets a paradigm for unlocking hidden quantum states in layered materials, offering a new dimension in design of quantum matter.

cond-mat.supr-con↗

arXiv2025

Giant plateau-like topological Hall effect controlled by tailoring the magnetic exchange stiffness in a kagome magnet

The ferrimagnet TbMn6Sn6 has attracted vast attention, because its pristine Mn kagome lattice with strong spin-orbit coupling and out-of-plane Tb-Mn exchange supports quantum-limit Chern topological magnetism which can be described by the simple spinless Haldane model. We unveil herein that engineering the kagome lattice through partial substitution of Mn with nonmagnetic Cr induces a striking structural reorganization-Cr preferentially concentrates within a single Mn layer per unit cell, reducing the crystal symmetry from the D6h point group to the C2. This tailored structure configuration gives rise to a plateau-like topological Hall effect (THE), achieving a record-breaking resistivity of 19.1 ohm cm among bulk systems. Complementary magnetic force microscopy measurements unveil a magnetic domain transition near 1 T at 180 K, aligning with the field-dependent phase diagram of the THE. Our direct visualization of the magnetic domain structure underscores the critical role of broken kagome lattice symmetry in generating distinct exchange stiffness between the two Mn layers. These findings establish a new paradigm for exploring exotic states in kagome topological magnets and provide a proof-of-principle strategy for unraveling the interplay between magnetism and emergent topological properties in kagome systems.

cond-mat.str-el↗

arXiv2025

Large unconventional anomalous Hall effect far above room temperature in epitaxial Fe$_3$Ga$_4$ films

Noncoplanar spin textures usually exhibit a finite scalar spin chirality (SSC) that can generate effective magnetic fields and lead to additional contributions to the Hall effect, namely topological or unconventional anomalous Hall effect (UAHE). Unlike topological spin textures (e.g., magnetic skyrmions), materials that exhibit fluctuation-driven SSC and UAHE are rare. So far, their realization has been limited to either low temperatures or high magnetic fields, both of which are unfavorable for practical applications. Identifying new materials that exhibit UAHE in a low magnetic field at room temperature is therefore essential. Here, we report the discovery of a large UAHE far above room temperature in epitaxial Fe$_3$Ga$_4$ films, where the fluctuation-driven SSC stems from the field-induced transverse-conical-spiral phase. Considering their epitaxial nature and the large UAHE stabilized at room temperature in a low magnetic field, Fe$_3$Ga$_4$ films represent an exciting, albeit rare, case of a promising candidate material for spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Site-selective polar compensation of Mott electrons in a double perovskite heterointerface

Double perovskite oxides (DPOs) with two transition metal ions ($A_2$$BB^\prime$O$_6$) offer a fascinating platform for exploring exotic physics and practical applications. Studying these DPOs as ultrathin epitaxial thin films on single crystalline substrates can add another dimension to engineering electronic, magnetic, and topological phenomena. Understanding the consequence of polarity mismatch between the substrate and the DPO would be the first step towards this broad goal. We investigate this by studying the interface between a prototypical insulating DPO Nd$_2$NiMnO$_6$ and a wide-band gap insulator SrTiO$_3$. The interface is found to be insulating in nature. By combining several experimental techniques and density functional theory, we establish a site-selective charge compensation process that occurs explicitly at the Mn site of the film, leaving the Ni sites inert. We further demonstrate that such surprising selectivity, which cannot be explained by existing mechanisms of polarity compensation, is directly associated with their electronic correlation energy scales. This study establishes the crucial role of Mott physics in polar compensation process and paves the way for designer doping strategies in complex oxides.

cond-mat.mtrl-sci↗

arXiv2024

All-electric mimicking synaptic plasticity based on the noncollinear antiferromagnetic device

Neuromorphic computing, which seeks to replicate the brain's ability to process information, has garnered significant attention due to its potential to achieve brain-like computing efficiency and human cognitive intelligence. Spin-orbit torque (SOT) devices can be used to simulate artificial synapses with non-volatile, high-speed processing and endurance characteristics. Nevertheless, achieving energy-efficient all-electric synaptic plasticity emulation using SOT devices remains a challenge. We chose the noncollinear antiferromagnetic Mn3Pt as spin source to fabricate the Mn3Pt-based SOT device, leveraging its unconventional spin current resulting from magnetic space breaking. By adjusting the amplitude, duration, and number of pulsed currents, the Mn3Pt-based SOT device achieves nonvolatile multi-state modulated by all-electric SOT switching, enabling emulate synaptic behaviors like excitatory postsynaptic potential (EPSP), inhibitory postsynaptic potential (IPSP), long-term depression (LTD) and the long-term potentiation (LTP) process. In addition, we show the successful training of an artificial neural network based on such SOT device in recognizing handwritten digits with a high recognition accuracy of 94.95 %, which is only slightly lower than that from simulations (98.04 %). These findings suggest that the Mn3Pt-based SOT device is a promising candidate for the implementation of memristor-based brain-inspired computing systems.

physics.app-ph↗

arXiv2024

Direct visualization of the impurity occupancy roadmap in Ni-substituted van der Waals ferromagnet Fe3GaTe2

Impurity substitution is a general strategy to study the intrinsic properties of a quantum material. However, when the target element has more than one Wyckoff position in the lattice, it is a big challenge but with extreme necessity to know the exact position and order of the occupancy of impurity atoms. Via comprehensive experimental and theoretical investigations, we establish herein the roadmap for Ni substitution in Fe3GaTe2, a van der Waals ferromagnet with the Curie temperature TC even reaching ~ 380 K. The results unambiguously reveal that in (Fe1-xNix)3GaTe2, Ni atoms initially form an van der Waals interlayer gap Ni3 sites when x < 0.1, and then gradually occupy the Fe2 sites. After replacing the Fe2 sites at x of ~ 0.75, they start to substitute for the Fe1 sites and eventually realize a full occupation at x = 1.0. Accordingly, TC and saturation magnetic moments of (Fe1-xNix)3GaTe2 both show nonlinear decrease, which is tightly tied to the Ni occupancy order as well as the different roles of Ni3, Fe1 and Fe2 sites in the spin Hamiltonian. The results not only yield fruitful insights into the essential roles of different Fe sites in producing the above room temperature high TC, but also set a paradigm for future impurity substitution study on other quantum materials.

cond-mat.str-el↗

arXiv2024

Memristive switching in the surface of a charge-density-wave topological semimetal

Owing to the outstanding properties provided by nontrivial band topology, topological phases of matter are considered as a promising platform towards low-dissipation electronics, efficient spin-charge conversion, and topological quantum computation. Achieving ferroelectricity in topological materials enables the non-volatile control of the quantum states, which could greatly facilitate topological electronic research. However, ferroelectricity is generally incompatible with systems featuring metallicity due to the screening effect of free carriers. In this study, we report the observation of memristive switching based on the ferroelectric surface state of a topological semimetal (TaSe4)2I. We find that the surface state of (TaSe4)2I presents out-of-plane ferroelectric polarization due to surface reconstruction. With the combination of ferroelectric surface and charge-density-wave-gapped bulk states, an electric switchable barrier height can be achieved in (TaSe4)2I-metal contact. By employing a multi-terminal grounding design, we manage to construct a prototype ferroelectric memristor based on (TaSe4)2I with on/off ratio up to 10^3, endurance over 10^3 cycles, and good retention characteristics. The origin of the ferroelectric surface state is further investigated by first-principles calculations, which reveals an interplay between ferroelectricity and band topology. The emergence of ferroelectricity in (TaSe4)2I not only demonstrates it as a rare but essential case of ferroelectric topological materials, but also opens new routes towards the implementation of topological materials in functional electronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Tuning magnetic and optical properties through strain in epitaxial LaCrO3 thin films

We report on the effect of epitaxial strain on magnetic and optical properties of perovskite LaCrO3 (LCO) single crystal thin films. Epitaxial LCO thin films are grown by pulsed laser deposition on proper choice of substrates to impose different strain states. A combined experimental and theoretical approach is used to demonstrate the direct correlation between lattice-strain and functional properties. The magnetization results show that the lattice anisotropy plays a critical role in controlling the magnetic behavior of LCO films. The strain induced tetragonality in the film lattice strongly affects the optical transitions and charge transfer gap in LCO. This study opens new possibilities to tailoring the functional properties of LCO and related materials by strain engineering in epitaxial growth.

cond-mat.mtrl-sci↗

arXiv2017

The effects of oxygen in spinel oxide Li1+xTi2-xO4-delta thin films

The evolution from superconducting LiTi2O4-delta to insulating Li4Ti5O12 thin films has been studied by precisely adjusting the oxygen pressure during the sample fabrication process. In the superconducting LiTi2O4-delta films, with the increase of oxygen pressure, the oxygen vacancies are filled, and the c-axis lattice constant decreases gradually. With the increase of the oxygen pressure to a certain critical value, the c-axis lattice constant becomes stable, which implies that the Li4Ti5O12 phase comes into being. The process of oxygen filling is manifested by the angular bright-field images of the scanning transmission electron microscopy techniques. The temperature of magnetoresistance changed from positive and negative shows a non-monotonous behavior with the increase of oxygen pressure. The theoretical explanation of the oxygen effects on the structure and superconductivity of LiTi2O4-delta has also been discussed in this work.

cond-mat.supr-con↗

arXiv2017

Anisotropic electron-phonon coupling in the spinel oxide superconductor

Among hundreds of spinel oxides, LiTi2O4 (LTO) is the only one that exhibits superconductivity (Tc ~13 K). Although the general electron-phonon coupling is still the main mechanism for electron pairing in LTO, unconventional behaviors such as the anomalous magnetoresistance, anisotropic orbital/spin susceptibilities, etc. reveal that both the spin and the orbital interactions should also be considered for understanding the superconductivity. Here, we investigate tunneling spectra of [111]-, [110]- and [001]-oriented high quality LTO thin films. Several bosonic modes in tunneling spectra are observed in the [111]- and [110]-oriented films but not in [001]-oriented ones, and these modes still exist at T = 2Tc and beyond the upper critical field, which are confirmed as stemming from electron-phonon interaction by DFT calculations. These modes only appear in special surface orientations, indicating that the electron-phonon coupling in LTO system is highly anisotropic and may be enhanced by orbital-related state. The anisotropic electron-phonon coupling should be taken seriously in understanding the nature of LTO superconductivity.

cond-mat.supr-con↗

arXiv2016

Crystallographic dependent transport properties and oxygen issue in superconducting LiTi2O4 thin films

A systematic study is performed on the spinel oxide, i.e. LiTi2O4 thin films oriented along [111]-, [110]-, and [001]-directions, to reveal the crystallographic dependence of transport properties. With decreasing temperature, the LiTi2O4 displays roughly identical onset temperatures of twofold symmetry of in-plane angular dependent magnetoresistivity (AMR) (at ~ 100 K), crossover from negative- to positive- magnetoresistance (at ~ 50 K), and coherence length in the superconducting state. While, the twofold symmetry in AMR itself suggests anisotropic electron scattering. The superconducting transition temperature (Tc) seems insensitive to the lattice parameter. Moreover, the spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) discloses that oxygen vacancies exist in the LiTi2O4 films. These oxygen vacancies cause the change of lattice but show little influence on superconductivity, differing from high-Tc cuprates where subtle variation of oxygen way lead to a significant change in superconductivity.

cond-mat.supr-con↗

arXiv2016

Magnetic interactions at the nanoscale in trilayer titanates LaTiO$_3$/SrTiO$_3$/YTiO$_3$

We report on the phase diagram of competing magnetic interactions at nanoscale in engineered ultra-thin trilayer heterostructures of LaTiO$_{3}$/SrTiO$_{3}$/YTiO$_{3}$, in which the interfacial inversion symmetry is explicitly broken. Combined atomic layer resolved scanning transmission electron microscopy with electron energy loss spectroscopy and electrical transport have confirmed the formation of a spatially separated two-dimensional electron liquid and high density two-dimensional localized magnetic moments at the LaTiO$_3$/SrTiO$_3$ and SrTiO$_3$/YTiO$_3$ interfaces, respectively. Resonant soft X-ray linear dichroism spectroscopy has demonstrated the presence of orbital polarization of the conductive LaTiO$_3$/SrTiO$_3$ and localized SrTiO$_3$/YTiO$_3$ electrons. Our results provide a route with prospects for exploring new magnetic interfaces, designing tunable two-dimensional $d$-electron Kondo lattice, and potential spin Hall applications.

cond-mat.str-el↗

arXiv2016

Mott Electrons in an Artificial Graphenelike Crystal of Rare-Earth Nickelate

Deterministic control over the periodic geometrical arrangement of the constituent atoms is the backbone of the material properties, that along with the interactions define the electronic and magnetic ground state. Following this notion, a bilayer of a prototypical rare-earth nickelate, NdNiO$_3$, combined with a dielectric spacer, LaAlO$_3$, has been layered along the pseudo cubic [111] direction. The resulting artificial graphene-like Mott crystal with magnetic 3$d$ electrons has antiferromagnetic correlations. In addition, a combination of resonant X-ray linear dichroism measurements and \textit{ab-initio} calculations reveal the presence of an ordered orbital pattern, which is unattainable in either bulk nickelates or nickelate based heterostructures grown along the [001] direction. These findings highlight another promising venue towards designing new quantum many-body states by virtue of geometrical engineering.

cond-mat.str-el↗