arXiv subjects

Qingfeng Zhan

Publications and source records attributed to Qingfeng Zhan.

17 recordsLinked to original sources

arXiv2026

Unconventional superconductivity in ScIr$_2$ chiral crystal with a kagome lattice

Materials with a kagome lattice host exotic quantum phenomena driven by the interplay between band topology, spin-orbit coupling, magnetism, and electronic correlations. While magnetism of kagome materials has been widely investigated, their unconventional superconductivity (SC) remains largely unexplored due to the limited availability of suitable materials. Here, we report evidence of unconventional SC in the ScIr$_{2-x}$Si$_{x}$ family by combining muon-spin spectroscopy measurements with band-structure calculations. The parent ScIr$_2$ undergoes a structural phase transition from a high-$T$ cubic- to a low-$T$ rhombohedral phase, while the Ir kagome layer remains, albeit slightly distorted. Although the structural transition is suppressed by Si substitution, the superconducting pairing of ScIr$_{2-x}$Si$_{x}$ remains well described by a two-gap model. Since at least one of the gaps has nodes, this indicates an unconventional SC. Its unconventional nature can be explained by the distinct flat bands occurring near the Fermi level, leading to strong electronic correlations in the ScIr$_{2-x}$Si$_{x}$ family. Moreover, the low-$T$ phase of ScIr$_2$ exhibits an Ir chiral chain; therefore, it can be classified as a topological chiral crystal. Overall, the unusual properties of the ScIr$_{2-x}$Si$_{x}$ family make it an interesting, albeit rare, system for studying the interplay between unconventional SC, flat bands, and chirality.

cond-mat.supr-con↗

arXiv2026

Strongly frustrated 2D magnetism in a 3D hexagonal perovskite

Exotic quantum phenomena are often found to occur in spin systems that exhibit low-dimensional magnetism. By combining nuclear magnetic resonance, neutron scattering, and muon-spin spectroscopy ($\mu$SR) techniques, we report a rare instance of strongly frustrated two-dimensional (2D) magnetism in a three-dimensional (3D) hexagonal perovskite. Here, Ba$_2$La$_2$MnTe$_2$O$_{12}$, a triangular-lattice magnet, is shown to undergo a magnetic transition at $T_\mathrm{N} \approx$ 4.4 K, below which the manganese moments form a 120$^{\circ}$ AFM order within the $ab$-plane, while staying disordered along the $c$-axis. This exotic ground state, which exhibits ideal 2D magnetism, is highly consistent with the persistently strong spin fluctuations and the large internal field distributions revealed by zero-field $\mu$SR. Further, the 2D magnetism also leads to a significant frustration, much larger than that of most known magnetically-ordered frustrated systems. Our work on Ba$_2$La$_2$MnTe$_2$O$_{12}$ not only challenges the interpretations of magnetic order in other 3D hexagonal perovskites, but it also provides insight into how the dimensionality affects the exotic magnetic states.

cond-mat.str-el↗

arXiv2026

Investigating the intrinsic anomalous Hall effect in MnPt3 topological semimetal

The cubic Cu$_3$Au-type $X$Pt$_3$ family ($X$ = V, Cr, and Mn) is a topological semimetal characterized by anti-crossing gapped nodal lines near the Fermi level, which give rise to significant Berry curvatures and thus to the anomalous Hall effect (AHE). Among the three members, CrPt$_3$ has been experimentally verified to exhibit a large anomalous Hall conductivity (AHC), while its counterparts MnPt$_3$ and VPt$_3$ remain largely unexplored. Here, a series of MnPt$_3$ thin films with varying thicknesses (20--70 nm) was epitaxially grown on the MgO substrates using magnetron sputtering and was systematically investigated by magnetization, electrical resistivity, and Hall resistivity measurements. MnPt$_3$ films undergo a ferromagnetic transition at a Curie temperature $T_\mathrm{C}$, which increases as the film thickness increases, reaching $\sim$ 344 K for the 70-nm-thick film. All the anomalous Hall transport properties of MnPt$_3$ films, including the resistivity, conductivity, and angle, exhibit a strong correlation with their magnetic properties. The scaling analysis suggests that the intrinsic Berry-curvature mechanism dominates the observed AHE, while the extrinsic contributions are much smaller. The intrinsic AHC increases as the film thickness increases, while the extrinsic AHC is thickness-independent. Such an enhanced intrinsic AHC in the MnPt$_3$ films is most likely attributed to the strain effect, implying that it serves as an effective method to tune the electronic band topology in the $X$Pt$_3$ topological semimetal.

cond-mat.str-el↗

arXiv2026

Engineering of Orbital Hybridization: An Exotic Strategy to Manipulate Orbital Current

Current-induced spin-orbit torque (SOT) plays a crucial role in the next-generation spin-orbitronics. Enhancing its efficiency is both fundamentally and practically interesting and remains a challenge to date. Recently, orbital counterparts of spin effects that do not rely on the spin-orbit coupling (SOC) have been found as an alternative mechanism to realize it. This work highlights the engineering of copper oxidation states for manipulating the orbital current and its torque in the CuO$_x$-based heterostructures. The orbital hybridization and thus the orbital-Rashba-Edelstein effect at the CuO$_x$/Cu interfaces are significantly enhanced by increasing the copper oxidation state, yielding a torque efficiency that is almost ten times larger than the conventional heavy metals. The Cu$_4$O$_3$/Cu interface, rather than the widely accepted CuO/Cu interface, is revealed to account for the enhanced SOT performance in the CuO$_x$-based heterostructures. In addition, the torque efficiency can be alternatively switched between high and low thresholds through the redox reaction. The current results establish an exotic and robust strategy for engineering the orbital current and SOT for spin-orbitronics, which applies to other weak-SOC materials.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous Hall effect and rich magnetic phase diagram of Mn$_{100-x}$Rh$_{x}$ epitaxial films

A series of Mn$_{100-x}$Rh$_x$ ($20 \le x \le 50$) thin films were epitaxially grown on the MgO substrate using magnetron sputtering technique, and were systematically investigated by magnetization, longitudinal electrical resistivity, and transverse Hall resistivity. After optimizing the growth conditions, phase-pure Mn$_{100-x}$Rh$_x$ films with a cubic CsCl-type structure were obtained, and their magnetic phase diagram was built. The manipulation of Rh content leads to a rich magnetic phase diagram, where three different regimes can be identified: for $x < 40$, Mn$_{100-x}$Rh$_x$ films undergo a ferromagnetic (FM) transition below $T_\mathrm{C} \approx$ 330-350 K; for $40 \le x \le 45$, in addition to the FM transition at $T_\mathrm{C} \approx$ 200 K, Mn$_{100-x}$Rh$_x$ films undergo a FM-to-antiferromagnetic (AFM) transition at $T_\mathrm{N} \approx$ 120 K; finally for $x > 45$, only one AFM transition at $T_\mathrm{N} \approx$ 150 K can be tracked. All the Mn$_{100-x}$Rh$_x$ films exhibit distinct anomalous Hall effect in their magnetically ordered state, which is most likely due to the intrinsic Berry-curvature mechanism. In addition, all the anomalous Hall transport properties, including the resistivity, conductivity, and angle exhibit a strong correlation with the magnetic properties of Mn$_{100-x}$Rh$_x$ films, which become most evident for $x$ = 35. Our systematic investigations suggest a strong correlation between magnetic properties and electronic band topology in Mn$_{100-x}$Rh$_x$ films, highlighting their great potential for AFM spintronics.

cond-mat.str-el↗

arXiv2025

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 N\'eel 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↗

arXiv2025

Discovery of nodal-line superconductivity in chiral crystals

Chiral crystals, whose key feature is the structural handedness, host exotic quantum phenomena driven by the interplay of band topology, spin-orbit coupling (SOC), and electronic correlations. Due to the limited availability of suitable chiral-crystal materials, their unconventional superconductivity (SC) remains largely unexplored. Here, we report the discovery of unconventional SC in the La(Rh,Ir)Si family of materials by combining muon-spin spectroscopy, band-structure calculations, and perturbation theory. This family, characterized by a double-helix chiral structure, hosts exotic multifold fermions that are absent in other topological chiral crystals. While LaRhSi behaves as a fully-gapped superconductor, the substitution of 4$d$-Rh by 5$d$-Ir significantly enhances the SOC and leads to the emergence of topological nodal-line SC in LaIrSi. The developed model shows that the nodal-line SC arises from an isotropic SOC with a specific strength. Such an exotic mechanism expands our conventional understanding of material candidates for unconventional SC, which typically rely on a significantly anisotropic SOC to promote the triplet pairing. Our work establishes a new type of phase diagram, which provides a comprehensive roadmap for identifying and engineering unconventional SC in chiral crystals. Furthermore, it calls for renewed investigations of unconventional SC in other widely studied superconductors with a chiral structure.

cond-mat.supr-con↗

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

Multiband superconductivity in the topological Kramers nodal-line semimetals

Recent band-structure calculations predict that the ruthenium-based ternary silicides are three-dimensional Kramers nodal line semimetals. Among them, NbRuSi and TaRuSi show bulk superconductivity (SC) below $T_c \sim 3$ K and 4 K, as well as spontaneous magnetic fields. The latter indicates the breaking of time-reversal symmetry and, thus, unconventional SC in both compounds. Previous temperature-dependent muon-spin spectroscopy studies failed to distinguish whether such compounds exhibit single-gap or multi-gap SC. Here, we report on systematic measurements of the field-dependent muon-spin relaxation rates in the superconducting state and on temperature-dependent electrical resistivity and specific heat under applied magnetic fields. Both the upper critical field and the field-dependent superconducting relaxation are well described by a two-band model. By combining our experimental results with numerical band-structure calculations, we provide solid evidence for multiband SC in NbRuSi and TaRuSi, and thus offer further insight into the unconventional- and topological nature of their superconductivity.

cond-mat.supr-con↗

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

Domino-like magnetic phase transition induced by a bias voltage in FeRh thin film

The first-order magnetic phase transition (MPT) usually happens with a very wide magnetic field span about tens of thousands Oersted which hinders its applications. In this work, we induce a domino-like MPT via introducing a bias voltage in FeRh thin film and thus realize a large narrowing of transition magnetic field span from 6*10^4 Oe to lower than 2*10^3 Oe at room temperature. Meanwhile, nonvolatile switchings between pure magnetic phases of FeRh at room temperature also can be realized by applying voltage pulses. The critical condition and phase diagram for domino-like MPTs are obtained in theory and our experiments support it well. Our works not only benefit the studies and applications of MPT-based devices but also are significant in the applications of the phase transition systems with resistance change.

cond-mat.mtrl-sci↗

arXiv2024

Nodeless superconductivity and topological nodal states in molybdenum carbide

The orthorhombic molybdenum carbide superconductor with $T_c$ = 3.2 K was investigated by muon-spin rotation and relaxation ($\mu$SR) measurements and by first-principle calculations. The low-temperature superfluid density, determined by transverse-field $\mu$SR, suggests a fully-gapped superconducting state in Mo$_2$C, with a zero-temperature gap $\Delta_0$ = 0.44 meV and a magnetic penetration depth $\lambda_0$ = 291 nm. The time-reversal symmetry is preserved in the superconducting state, as confirmed by the absence of an additional muon-spin relaxation in the zero-field $\mu$SR spectra. Band-structure calculations indicate that the density of states at the Fermi level is dominated by the Mo $4d$-orbitals, which are marginally hybridized with the C $2p$-orbitals over a wide energy range. The symmetry analysis confirms that, in the absence of spin-orbit coupling (SOC), Mo$_2$C hosts twofold-degenerate nodal surfaces and fourfold-degenerate nodal lines. When considering SOC, the fourfold-degenerate nodal lines cross the Fermi level and contribute to the electronic properties. Our results suggest that, similarly to other phases of carbides, also the orthorhombic transition-metal carbides host topological nodal states and may be potential candidates for future studies of topological superconductivity.

cond-mat.supr-con↗

arXiv2024

Spin order and dynamics in the topological rare-earth germanide semimetals

The $RE$Al(Si,Ge) ($RE$ = rare earth) family, known to break both the inversion- and time-reversal symmetries, represents one of the most suitable platforms for investigating the interplay between correlated-electron phenomena and topologically nontrivial bands. Here, we report on systematic magnetic, transport, and muon-spin rotation and relaxation ($\mu$SR) measurements on (Nd,Sm)AlGe single crystals, which exhibit antiferromagnetic (AFM) transitions at $T_\mathrm{N} = 6.1$ and 5.9 K, respectively. In addition, NdAlGe undergoes also an incommensurate-to-commensurate ferrimagnetic transition at 4.5 K. Weak transverse-field $\mu$SR measurements confirm the AFM transitions, featuring a $\sim$90 % magnetic volume fraction. In both cases, zero-field (ZF) $\mu$SR measurements reveal a more disordered internal field distribution in NdAlGe than in SmAlGe, reflected in a larger transverse muon-spin relaxation rate $\lambda^\mathrm{T}$ at $T \ll T_\mathrm{N}$. This may be due to the complex magnetic structure of NdAlGe, which undergoes a series of metamagnetic transitions in an external magnetic field, while SmAlGe shows only a robust AFM order. In NdAlGe, the topological Hall effect (THE) appears between the first and the second metamagnetic transitions for $H \parallel c$, while it is absent in SmAlGe. Such THE in NdAlGe is most likely attributed to the field-induced topological spin textures. The longitudinal muon-spin relaxation rate $\lambda^\mathrm{L}(T)$, diverges near the AFM order, followed by a clear drop at $T < T_\mathrm{N}$. In the magnetically ordered state, spin fluctuations are significantly stronger in NdAlGe than in SmAlGe. In general, our longitudinal-field $\mu$SR data indicate vigorous spin fluctuations in NdAlGe, thus providing valuable insights into the origin of THE and of the possible topological spin textures in $RE$Al(Si,Ge) Weyl semimetals.

cond-mat.str-el↗

arXiv2024

Enhanced torque efficiency in ferromagnetic multilayers by introducing naturally oxidized Cu

Spin-orbit torque (SOT) in the heavy elements with a large spin-orbit coupling (SOC) has been frequently used to manipulate the magnetic states in spintronic devices. Recent theoretical works have predicted that the surface oxidized light elements with a negligible SOC can yield a sizable orbital torque (OT), which plays an important role in switching the magnetization. Here, we report anomalous-Hall-resistance and harmonic-Hall-voltage measurements on perpendicularly magnetized Ta/Cu/[Ni/Co]$_5$/Cu-CuO$_x$ multilayers. Both torque efficiency and spin-Hall angle of these multilayers are largely enhanced by introducing a naturally oxidized Cu-CuO$_x$ layer, where the SOC is negligible. Such an enhancement is mainly due to the collaborative driven of the SOT from the Ta layer and the OT from the Cu/CuO$_x$ interface, and can be tuned by controlling the thickness of Cu-CuO$_x$ layer. Compared to the Cu-CuO$_x$-free multilayers, the maximum torque efficiency and spin-Hall angle were enhanced by a factor of ten, larger than most of the reported values in the other heterostructures.

cond-mat.mtrl-sci↗

arXiv2023

Absence of topological Hall effect in Fe$_x$Rh$_{100-x}$ epitaxial films: revisiting their phase diagram

A series of Fe$_x$Rh$_{100-x}$ ($30 \leq x \leq 57$) films were epitaxially grown using magnetron sputtering, and were systematically studied by magnetization-, electrical resistivity-, and Hall resistivity measurements. After optimizing the growth conditions, phase-pure Fe$_{x}$Rh$_{100-x}$ films were obtained, and their magnetic phase diagram was revisited. The ferromagnetic (FM) to antiferromagnetic (AFM) transition is limited at narrow Fe-contents with $48 \leq x \leq 54$ in the bulk Fe$_x$Rh$_{100-x}$ alloys. By contrast, the FM-AFM transition in the Fe$_x$Rh$_{100-x}$ films is extended to cover a much wider $x$ range between 33 % and 53 %, whose critical temperature slightly decreases as increasing the Fe-content. The resistivity jump and magnetization drop at the FM-AFM transition are much more significant in the Fe$_x$Rh$_{100-x}$ films with $\sim$50 % Fe-content than in the Fe-deficient films, the latter have a large amount of paramagnetic phase. The magnetoresistivity (MR) is rather weak and positive in the AFM state, while it becomes negative when the FM phase shows up, and a giant MR appears in the mixed FM- and AFM states. The Hall resistivity is dominated by the ordinary Hall effect in the AFM state, while in the mixed state or high-temperature FM state, the anomalous Hall effect takes over. The absence of topological Hall resistivity in Fe$_{x}$Rh$_{100-x}$ films with various Fe-contents implies that the previously observed topological Hall effect is most likely extrinsic. We propose that the anomalous Hall effect caused by the FM iron moments at the interfaces nicely explains the hump-like anomaly in the Hall resistivity. Our systematic investigations may offer valuable insights into the spintronics based on iron-rhodium alloys.

cond-mat.mtrl-sci↗

arXiv2022

Anomalous anisotropy of spin current in a cubic spin source with noncollinear antiferromagnetism

Cubic materials host high crystal symmetry and hence are not expected to support anisotropy in transport phenomena. In contrast to this common expectation, here we report an anomalous anisotropy of spin current can emerge in the (001) film of Mn${_3}$Pt, a noncollinear antiferromagnetic spin source with face-centered cubic structure. Such spin current anisotropy originates from the intertwined time reversal-odd ($T$-odd) and time reversal-even ($T$-even) spin Hall effects. Based on symmetry analyses and experimental characterizations of the current-induced spin torques in Mn${_3}$Pt-based heterostructures, we find that the spin current generated by Mn${_3}$Pt (001) exhibits exotic dependences on the current direction for all the spin components, deviating from that in conventional cubic systems. We also demonstrate that such an anisotropic spin current can be used to realize low-power spintronic applications such as the efficient field-free switching of the perpendicular magnetizations.

cond-mat.mtrl-sci↗

arXiv2013

Fabrication, properties, and applications of flexible magnetic films

Flexible magnetic devices, i.e., magnetic devices fabricated on flexible substrates, are very attractive in application of detecting magnetic field in arbitrary surface, non-contact actuators, and microwave devices due to the stretchable, biocompatible, light-weight, portable, and low cost properties. Flexible magnetic films are essential for the realization of various functionalities of flexible magnetic devices. To give a comprehensive understanding for flexible magnetic films and related devices, we have reviewed recent advances in the studies of flexible magnetic films including fabrication methods, magnetic and transport properties of flexible magnetic films, and their applications in magnetic sensors, actuators, and microwave devices. Three typical methods were introduced to prepare the flexible magnetic films. Stretching or bending the flexible magnetic films offers a good way to apply mechanical strain on magnetic films, so that magnetic anisotropy, exchanged bias, coercivity, and magnetoresistance can be effectively manipulated. Finally, a series of examples were shown to demonstrate the great potential of flexible magnetic films for future applications.

cond-mat.mtrl-sci↗