arXiv subjects

Aifeng Wang

Publications and source records attributed to Aifeng Wang.

At least 37 records · Page 2Linked to original sources

arXiv2024

Large Nernst Effect in a layered metallic antiferromagnet EuAl$_2$Si$_2$

The large Nernst effect is advantageous for developing transverse Nernst thermoelectric generators or Ettingshausen coolers within a single component, avoiding the complexity of electron- and hole-modules in longitudinal Seebeck thermoelectric devices. We report a large Nernst signal reaching 130 uV/K at 8 K and 13 T in the layered metallic antiferromagnet EuAl$_2$Si$_2$. Notably, this large transverse Nernst thermopower is two orders of magnitude greater than its longitudinal counterpart. The Nernst coefficient peaks around 4 K and 8 K at 3 T and 13 T, respectively. At similar temperatures, both the Hall coefficient and the Seebeck signal change sign. Additionally, nearly compensated electron- and hole-like carriers with high mobility ($\sim$ 4000 cm$^2$/Vs at 4 K) are revealed from the magnetoconductivity. These findings suggest that the large Nernst effect and vanishing Seebeck thermopower in EuAl$_2$Si$_2$ are due to the compensated electron- and hole-like bands, along with the high mobility of the Weyl band near the Fermi level. Our results underscore the importance of band compensation and topological fermiology in achieving large Nernst thermopower and exploring potential Nernst thermoelectric applications at low temperatures.

cond-mat.mtrl-sci↗

arXiv2024

Probing critical spin fluctuations with a composite magnetoelectric method: A case study on a Kitaev spin liquid candidate Na$_3$Co$_2$SbO$_6$

In correlated quantum materials, divergent critical fluctuations near the quantum critical point are often closely associated with exotic quantum phases of matter, such as unconventional superconductivity and quantum spin liquids. Here we present a simple yet highly sensitive composite magnetoelectric (ME) method for detecting the critical spin fluctuations in quantum magnets. The ME signal is proportional the magnetostriction coefficient, which directly probes the product of magnetization and spin-spin correlation. As a demonstration, the composite ME method is applied to a Kitaev quantum spin liquid candidate Na$_3$Co$_2$SbO$_6$, which shows signs of magnetic field-induced quantum criticality. Notably, the ME signal prominently diverges at the magnetic field-induced tricritical points, particularly at a tricritical point that lies in close proximity to a zero-temperature quantum critical point. A crucial aspect of these tricritical points is their tunability through the modification of the in-plane magnetic field's direction. The direction of magnetic field can thus serve as a handful yet important tuning parameter, alongside pressure and chemical doping, for searching quantum critical points in quantum magnets with pronounced magnetic anisotropy.

cond-mat.str-el↗

arXiv2024

Evidence for distortion-induced local electric polarization in $α$-RuCl$_3$

The spin-orbit assisted Mott insulator $α$-RuCl$_3$ is a prime candidate for material realization of the Kitaev quantum spin liquid. While little attention has been paid to charge degrees of freedom, charge effects, such as electric polarization, may arise in this system. Here, we report distortion-induced local electric polarization in $α$-RuCl$_3$ as evidenced by single-crystal X-ray diffraction, second harmonic generation (SHG) and dielectric measurements. The SHG signal appears at room temperature and develops substantially in the Kitaev paramagnetic state when short-range spin correlations come into play. Despite sizable pyroelectric currents in the Kitaev paramagnetic state, the absence of hysteresis in electric field-dependent polariza-tion (P-E) points to the short-range nature of electric polarization. This localized electric polarization is likely the result of distortion-induced charge dimerization, achieved through virtual hopping-induced charge redistribution. In addition, the electric polarization is boosted by short-range spin correlations via spin-phonon coupling in the Kitaev paramagnetic state. Our results emphasize the importance of charge degrees of freedom in $α$-RuCl$_3$, which establish a novel platform to investi-gate charge effects in Kitaev materials.

cond-mat.str-el↗

arXiv2024

Annealing-tunable charge density wave in the kagome antiferromagnet FeGe

The unprecedented phenomenon that a charge density wave (CDW) emerges inside the antiferromagnetic (AFM) phase indicates an unusual CDW mechanism associated with magnetism in FeGe. Here, we demonstrate that both the CDW and magnetism of FeGe can be effectively tuned through post-growth annealing treatments. Instead of the short-range CDW reported earlier, a long-range CDW order is realized below 110 K in single crystals annealed at \SI{320}{\degreeCelsius} for over 48 h. The CDW and AFM transition temperatures appear to be inversely correlated with each other. The entrance of the CDW phase significantly reduces the critical field of the spin-flop transition, whereas the CDW transition remains stable against minor variations in magnetic orders such as annealing-induced magnetic clusters and spin-canting transitions. Single-crystal x-ray diffraction measurements reveal substantial disorder on the Ge1 site, which is characterized by displacement of the Ge1 atom from Fe$_3$Ge layer along the $c$ axis and can be reversibly modified by the annealing process. The observed annealing-tunable CDW and magnetic orders can be well understood in terms of disorder on the Ge1 site. Our study provides a vital starting point for the exploration of the unconventional CDW mechanism in FeGe and of kagome materials in general.

cond-mat.str-el↗

arXiv2024

Pressure tunable magnetic skyrmion phase in Co8Zn8Mn4 single crystals

In a magnetic skyrmion phase, magnetic moments form vortex-like topological textures which are of both fundamental and industrial interests. In $β$-Mn-type Co-Zn-Mn alloys, chrial magnetic skyrmions emerge above room temperature, providing a unique system for studying the skrymion physics and exploring spintronics applications. However, the magnetic skyrmion phase is typically confined in a narrow and limited temperature ($T$) and magnetic field ($H$) range. Here, we demonstrate that hydrostatic pressure can expand the skyrmion phase in the $T-H$ phase diagram of single-crystalline Co$_8$Zn$_8$Mn$_4$. At ambient pressure, signatures of skyrmions are seen within $T\sim302-308$ K and $H\sim50-100$ Oe. Applying a moderate pressure of 6 kbar extends this range to $T\sim300-310$ K and $H\sim50-150$ Oe. However, further escalation of pressure to 10 kbar results in a slight contraction of the skyrmion phase. These findings underscore the sensitivity of the skyrmion phase in Co$_8$Zn$_8$Mn$_4$ to external pressures, and hint at the potential of strain engineering, particularly in $β$-Mn-type Co-Zn-Mn thin films, as a promising avenue to customize the skyrmion phase.

cond-mat.mtrl-sci↗

arXiv2024

Evidence for Unfolded Fermi Surfaces in the Charge-Density-Wave State of Kagome Metal FeGe Revealed by de Haas-van Alphen Effect

The antiferromagnetic kagome lattice compound FeGe has been revealed to host an emergent charge-density-wave (CDW) state which manifests complex interplay between the spin, charge and lattice degrees of freedom. Here, we present a comprehensive study of the de Haas-van Alphen effect by measuring torque magnetometry under magnetic fields up to 45.2 T to map Fermi surfaces in this unusual CDW state. For field along the $c$ direction, we resolve four cyclotron orbits; the largest one roughly corresponding to the area of the 2$\times$2 folded Brillouin zone. Three smaller orbits are characterized by light effective cyclotron masses range from (0.18-0.30) $m_e$. Angle-resolved measurements identify one Fermi surface segment with weak anisotropy. Combined with band structure calculations, our results suggest that features of unfolded Fermi surfaces are robust against CDW reconstruction, corroborating the novel effect of a short-ranged CDW on the electronic structure.

cond-mat.str-el↗

arXiv2023

Electrical and thermal transport properties of kagome metals AV$_3$Sb$_5$ (A=K, Rb, Cs)

The interplay between lattice geometry, band topology and electronic correlations in the newly discovered kagome compounds AV$_3$Sb$_5$ (A=K, Rb, Cs) makes this family a novel playground to investigate emergent quantum phenomena, such as unconventional superconductivity, chiral charge density wave and electronic nematicity. These exotic quantum phases naturally leave nontrivial fingerprints in transport properties of AV$_3$Sb$_5$, both in electrical and thermal channels, which are prominent probes to uncover the underlying mechanisms. In this brief review, we highlight the unusual electrical and thermal transport properties observed in the unconventional charge ordered state of AV3Sb5, including giant anomalous Hall, anomalous Nernst, ambipolar Nernst and anomalous thermal Hall effects. Connections of these anomalous transport properties to time-reversal symmetry breaking, topological and multiband fermiology, as well as electronic nematicity, are also discussed. Finally, a perspective together with challenges of this rapid growing field are given.

cond-mat.str-el↗

arXiv2023

Coexistence of Dirac fermion and charge density wave in square-net-based semimetal LaAuSb2

We report a comprehensive study of magnetotransport properties, angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations on self-flux grown LaAuSb$_2$ single crystals. Resistivity and Hall measurements reveal a charge density wave (CDW) transition at 77 K. MR and de Haas-Van Alphen (dHvA) measurements indicate that the transport properties of LaAuSb$_2$ are dominated by Dirac fermions that arise from Sb square nets. ARPES measurements and DFT calculations reveal an electronic structure with a common feature of the square-net-based topological semimetals, which is in good agreement with the magnetotransport properties. Our results indicate the coexistence of CDW and Dirac fermion in LaAuSb$_2$, both of which are linked to the bands arising from the Sb-square net, suggesting that the square net could serve as a structural motif to explore various electronic orders.

cond-mat.mtrl-sci↗

arXiv2023

Photoemission Evidence of a Novel Charge Order in Kagome Metal FeGe

A charge order has been discovered to emerge deep into the antiferromagnetic phase of the kagome metal FeGe. To study its origin, the evolution of the low-lying electronic structure across the charge order phase transition is investigated with angle-resolved photoemission spectroscopy. We do not find signatures of nesting between Fermi surface sections or van-Hove singularities in zero-frequency joint density of states, and there are no obvious energy gaps at the Fermi level, which exclude the nesting mechanism for the charge order formation in FeGe. However, two obvious changes in the band structure have been detected, i.e., one electron-like band around the K point and another one around the A point move upward in energy position when the charge order forms. These features can be well reproduced by our density-functional theory calculations, where the charge order is primarily driven by magnetic energy saving via large dimerizations of a quarter of Ge1-sites (in the kagome plane) along the c-axis. Our results provide strong support for this novel charge order formation mechanism in FeGe, in contrast to the conventional nesting mechanism.

cond-mat.str-el↗

arXiv2023

Comprehensive investigation of Quantum Oscillations in Semimetal Using an ac Composite Magnetoelectric Technique with Ultrahigh Sensitivity

Quantum oscillation (QO), a physical phenomenon that reflects the characteristics of the Fermi surface and transport fermions, has been extensively observed in metals and semimetals through various approaches, like magnetostriction, magnetization, resistivity, and thermoelectric power. However, only some allowed oscillation frequencies can be revealed by each individual method, particularly in semimetals with intricate Fermi pockets and associated magnetic breakdown phenomena. In this paper, we present the application of an ac composite magnetoelectric (ME) technique to measure the QOs of a topological nodal-line semimetal, ZrSiS, which possesses six fundamental QO frequencies. By employing the ME technique with a maximum magnetic field of 13 T and a minimum temperature of 2 K, we are able to capture all the fundamental frequencies and most of the permissible magnetic breakdown frequencies. In comparison, some of the frequencies were missing in the aforementioned four methods under identical measurement conditions. Remarkably, a series of magnetic breakdown frequencies around 8000 T were revealed even in a magnetic field as low as 7.5 T. These findings highlight the ME technique as an ultrahigh-sensitive tool for studying Dirac Fermions and other topological semimetals with complex Fermi surfaces.

cond-mat.str-el↗

arXiv2023

Quantum metric-induced nonlinear transport in a topological antiferromagnet

The Berry curvature and quantum metric are the imaginary part and real part, respectively, of the quantum geometric tensor which characterizes the topology of quantum states. The former is known to generate a zoo of important discoveries such as quantum Hall effect and anomalous Hall effect (AHE), while the consequences of the quantum metric have rarely been probed by transport. In this work, we observed quantum metric induced nonlinear transport, including both nonlinear AHE and diode-like nonreciprocal longitudinal response, in thin films of a topological antiferromagnet, MnBi$_2$Te$_4$. Our observation reveals that the transverse and longitudinal nonlinear conductivities reverse signs when reversing the antiferromagnetic order, diminish above the Néel temperature, and are insensitive to disorder scattering, thus verifying their origin in the band structure topology. They also flip signs between electron and hole-doped regions, in agreement with theoretical calculations. Our work provides a pathway to probe the quantum metric through nonlinear transport and to design magnetic nonlinear devices.

cond-mat.mes-hall↗

arXiv2023

Spin-phonon scattering-induced low thermal conductivity in a van der Waals layered ferromagnet Cr$_2$Si$_2$Te$_6$

Layered van der Waals (vdW) magnets are prominent playgrounds for developing magnetoelectric, magneto-optic and spintronic devices. In spintronics, particularly in spincaloritronic applications, low thermal conductivity ($κ$) is highly desired. Here, by combining thermal transport measurements with density functional theory calculations, we demonstrate low $κ$ down to 1 W m$^{-1}$ K$^{-1}$ in a typical vdW ferromagnet Cr$_2$Si$_2$Te$_6$. In the paramagnetic state, development of magnetic fluctuations way above $T_\mathrm{c}=$ 33 K strongly reduces $κ$ via spin-phonon scattering, leading to low $κ\sim$ 1 W m$^{-1}$ K$^{-1}$ over a wide temperature range, in comparable to that of amorphous silica. In the magnetically ordered state, emergence of resonant magnon-phonon scattering limits $κ$ below $\sim$ 2 W m$^{-1}$ K$^{-1}$, which would be three times larger if magnetic scatterings were absent. Application of magnetic fields strongly suppresses the spin-phonon scattering, giving rise to large enhancements of $κ$. Our calculations well capture these complex behaviours of $κ$ by taking the temperature- and magnetic-field-dependent spin-phonon scattering into account. Realization of low $κ$ which is easily tunable by magnetic fields in Cr$_2$Si$_2$Te$_6$, may further promote spincaloritronic applications of vdW magnets. Our theoretical approach may also provide a generic understanding of spin-phonon scattering, which appears to play important roles in various systems.

cond-mat.mtrl-sci↗

arXiv2023

Charge fluctuations above $T_\mathrm{CDW}$ revealed by glasslike thermal transport in kagome metals $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs)

We present heat capacity, electrical and thermal transport measurements of kagome metals $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs). In all three compounds, development of short-range charge fluctuations above the charge density wave (CDW) transition temperature $T_\mathrm{CDW}$ strongly scatters phonons via electron-phonon coupling, leading to glasslike phonon heat transport, i.e., phonon thermal conductivity decreases weakly upon cooling. Once the long-range charge order sets in below $T_\mathrm{CDW}$, short-range charge fluctuations are quenched, and the typical Umklapp scattering dominated phonon heat transport is recovered. The charge-fluctuations-induced glasslike phonon thermal conductivity implies sizable electron-phonon coupling in $A$V$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2023

Electrical and thermal transport properties of kagome metals $A$Ti$_3$Bi$_5$ ($A$ = Rb, Cs)

We report electrical and thermal transport properties of single crystalline kagome metals $A$Ti$_3$Bi$_5$ ($A$ = Rb, Cs). Different from the structural similar kagome superconductors $A$V$_3$Sb$_5$, no charge density wave instabilities are found in $A$Ti$_3$Bi$_5$. At low temperatures below 5 K, signatures of superconductivity appear in $A$Ti$_3$Bi$_5$ as seen in magnetization measurements. However, bulk superconductivity is not evidenced by specific heat results. Similar to $A$V$_3$Sb$_5$, $A$Ti$_3$Bi$_5$ show nonlinear magnetic field dependence of the Hall effect below about 70 K, pointing to a multiband nature. Unlike $A$V$_3$Sb$_5$ in which phonons and electron-phonon coupling play important roles in thermal transport, the thermal conductivity in $A$Ti$_3$Bi$_5$ is dominated by electronic contributions. Moreover, our calculated electronic structure suggests that van Hove singularities are sitting well above the Fermi energy. Compared with $A$V$_3$Sb$_5$, the absence of charge orders in $A$Ti$_3$Bi$_5$ is closely associated with minor contributions from electron-phonon coupling and/or van Hove singularities.

cond-mat.supr-con↗

arXiv2023

Anomalous Nernst effect in a ferrimagnetic nodal-line semiconductor Mn$_3$Si$_2$Te$_6$

In the ferrimagnetic nodal-line semiconductor Mn$_3$Si$_2$Te$_6$, colossal magnetoresistance (CMR) arises below $T_\mathrm{c}=78$ K due to the interplay of magnetism and topological nodal-line fermiology. The Berry curvature associated with the topological nodal-line is expected to produce an anomalous Nernst effect. Here, we present sizable anomalous Nernst signal in Mn$_3$Si$_2$Te$_6$ below $T_\mathrm{c}$. In the low-magnetic-field region where CMR is most apparent, the scaling ratio between the Nernst signal and magnetization is significantly enhanced compared to that in conventional magnetic materials. The enhanced Nernst effect and CMR likely share the same mechanisms, which are closely linked to the nodal-line topology.

cond-mat.str-el↗

arXiv2022

Multiband effects in thermoelectric and electrical transport properties of kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs)

We studied the effects of multiband electronic structure on the thermoelectric and electrical transport properties in the normal state of kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs). In all three members, the multiband nature is manifested by sign changes in the temperature dependence of the Seebeck and Hall resistivity, together with sublinear response of the isothermal Nernst and Hall effects to external magnetic fields in the charge ordered state. Moreover, ambipolar transport effects appear ubiquitously in all three systems, giving rise to sizable Nernst signal. Finally, possible origins of the sign reversal in the temperature dependence of the Hall effect are discussed.

cond-mat.supr-con↗

arXiv2022

Suppression of Superconductivity and Nematic Order in Fe$_{1-y}$Se$_{1-x}$S$_x$ (0$\leq$$x$$\leq$1, $y$$\leq$0.1) Crystals by Anion Height Disorder

Connections between crystal chemistry and critical temperature $T_c$ have been in the focus of superconductivity, one of the most widely studied phenomena in physics, chemistry and materials science alike. In most Fe-based superconductors, materials chemistry and physics conspire so that $T_c$ correlates with the average anion height above the Fe plane, i. e. with the geometry of the FeAs4 or FeCh4 (Ch = Te, Se, or S) tetrahedron. By synthesizing Fe$_{1-y}$Se$_{1-x}$S$_x$ (0$\leq$$x$$\leq$1, $y$$\leq$0.1), we find that in alloyed crystals $T_c$ is not correlated with the anion height like it is for most other Fe superconductors. Instead, changes in $T_c$($x$) and tetragonal-to-orthorombic (nematic) transition $T_s$($x$) upon cooling are correlated with disorder in Fe vibrations in the direction orthogonal to Fe planes, along the crystallographic c-axis. The disorder stems from the random nature of S substitution, causing deformed Fe(Se,S)4 tetrahedra with different Fe-Se and Fe-S bond distances. Our results provide evidence of $T_c$ and $T_s$ suppression by disorder in anion height. The connection to local crystal chemistry may be exploited in computational prediction of new superconducting materials with Fe/S building blocks.

cond-mat.supr-con↗

arXiv2022

Non-reciprocal charge transport in an intrinsic magnetic topological insulator MnBi2Te4

Symmetries, quantum geometries and electronic correlations are among the most important ingredients of condensed matters, and lead to nontrivial phenomena in experiments, for example, non-reciprocal charge transport. Here we report the non-reciprocal charge transport in the intrinsic magnetic topological insulator MnBi2Te4. The current direction relevant resistance is observed at chiral edges, which is magnetically switchable, edge position sensitive and septuple layer number controllable. Applying gate voltage can effectively tune the non-reciprocal resistance. The observation and manipulation of non-reciprocal charge transport indicate the fundamental role of chirality in charge transport of MnBi2Te4, and pave ways to develop van der Waals spintronic devices by chirality engineering.

cond-mat.mes-hall↗