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Zhi-An Ren

Publications and source records attributed to Zhi-An Ren.

At least 19 recordsLinked to original sources
arXiv2026

Surface-switchable nonreciprocity protected by Fermi arcs in Weyl semimetal TaAs

Weyl semimetals host topologically protected surface states, known as Fermi arcs, which connect bulk Weyl nodes in momentum space. Both bulk Weyl nodes and Fermi arcs are anticipated to be chiral. The chirality of bulk bands has been confirmed through observations of the chiral anomaly and Weyl orbits. In contrast, despite their discovery more than a decade ago, the chiral nature of Fermi arcs has remained unresolved. Here we report Fermi-arc-induced nonlinear transport in the archetypal Weyl semimetal TaAs. Using focused ion beam techniques, we fabricated micro-scale devices that enable simultaneous transport measurements on opposing topological surfaces. While linear transport remains dominated by bulk conduction, nonlinear transport uncovers surface-specific contributions, including an exceptionally large third-order nonreciprocal response that exceeds conventional expectations and highlights the crucial role of the singular arc endpoints. Our findings unambiguously demonstrate the chiral nature of Fermi arcs and establish nonlinear transport as a direct probe of these topological surface states. By revealing a surface-switchable, room-temperature nonlinear response that is topologically protected, this work introduces a new functionality in Weyl semimetals. Given the abundance of natural materials predicted to host topological semimetal states, these results open opportunities for exploring nonlinear transport phenomena and device concepts across a broad class of systems.

cond-mat.mes-hall↗
arXiv2025

Zero resistance when metals mixed with insulators

A false zero resistance behavior was observed during our study on the search of superconductivity in Ge-doped GaNb4Se8. This zero resistance was proved to be caused by open-circuit in multi-phase samples comprised of metals and insulators by measuring with four-probe method. The evidence strongly suggests that the reported superconductivity in hydrides should be carefully re-checked.

cond-mat.supr-con↗
arXiv2024

Thermodynamic signatures of a potential Fulde-Ferrell-Larkin Ovchinnikov state in the isotropic superconductor Ti4Ir2O

This study presents an investigation of the isotropic superconductor Ti4Ir2O, focusing on its magnetic field vs. temperature phase diagram near and above the Pauli limit for superconductivity. The data exhibits characteristic features that align with the formation of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, including a deviation of the upper critical field line from the standard WHH model upon approaching the Pauli limiting field and a phase transition line separating the ordinary low-field superconducting phase from an unusual high field superconducting state above the Pauli limit. We discuss why the upper critical field line in this isotropic superconductor can approach the Pauli limit, necessitating a particularly high orbital limit for superconductivity. The research also identifies peculiar features in the specific heat data set taken at 0.6 K, which may be an example of the segmentation of the FFLO phase with multiple q phases. The findings provide an explanation for the violation of the Pauli limit in this superconductor, adding a fully isotropic superconductor to the list of potential FFLO superconductors and providing new insights for contemporary theories of the FFLO state.

cond-mat.supr-con↗
arXiv2023

Emergence of flat bands and their impact on superconductivity of Mo$_5$Si$_{3-x}$P$_x$

The first-principles calculations and measurements of the magnetic penetration depths, the upper critical field, and the specific heat were performed for a family of Mo$_5$Si$_{3-x}$P$_x$ superconducotrs. First-principles calculations suggest the presence of a flat band dispersion, which gradually shifts to the Fermi level as a function of phosphorus doping $x$. The flat band approaches the Fermi level at $x\simeq 1.3$, thus separating Mo$_5$Si$_{3-x}$P$_x$ between the purely steep band and the steep band/flat band superconducting regimes. The emergence of flat bands lead to an abrupt change of nearly all the superconducting quantities. In particular, a strong reduction of the coherence length $ξ$ and enhancement of the penetration depth $λ$ result in nearly factor of three increase of the Ginzburg-Landau parameter $κ=λ/ξ$ (from $κ\simeq 25$ for $x\lesssim 1.2$ to $κ\simeq 70$ for $x\gtrsim 1.4$) thus initiating the transition of Mo$_5$Si$_{3-x}$P$_x$ from a moderate to an extreme type-II superconductivity.

cond-mat.supr-con↗
arXiv2023

Superconductivity in Mo$_4$Ga$_{20}$As with Endohedral Gallium Clusters

We report the discovery and detailed investigation of superconductivity in Mo$_4$Ga$_{20}$As. Mo$_4$Ga$_{20}$As crystallizes in the space group of $I4/m$ (No. 87), with lattice parameters $a$ = 12.86352 Åand $c$ = 5.30031 Å. The resistivity, magnetization, and specific heat data reveal Mo$_4$Ga$_{20}$As to be a type-II superconductor with $T_c$ = 5.6 K. The upper and lower critical fields are estimated to be 2.78 T and 22.0 mT, respectively. In addition, electron-phonon coupling in Mo$_4$Ga$_{20}$As is possibly stronger than the BCS weak-coupling limit. First-principles calculations suggest the Fermi level being dominated by the Mo-4$d$ and Ga-4$p$ orbitals.

cond-mat.supr-con↗
arXiv2022

Superconductivity Induced by Site-Selective Arsenic Doping in Mo$_5$Si$_3$

Arsenic doping in silicides has been much less studied compared with phosphorus. In this study, superconductivity is successfully induced by As doping in Mo$_5$Si$_3$. The superconducting transition temperature ($T_c$) reaches 7.7 K, which is higher than those in previously known W$_5$Si$_3$-type superconductors. Mo$_5$Si$_2$As is a type-II BCS superconductor with upper and lower critical fields of 6.65 T and 22.4 mT, respectively. In addition, As atoms are found to selectively take the 8$h$ sites in Mo$_5$Si$_2$As. The emergence of superconductivity is possibly due to the shift of Fermi level as a consequence of As doping, as revealed by the specific heat measurements and first-principles calculations. Our work provides not only another example of As doping, but also a practical strategy to achieve superconductivity in silicides through Fermi level engineering.

cond-mat.supr-con↗
arXiv2022

Strong-Coupling Superconductivity with $T_c$ $\sim$ 10.8 K Induced by P Doping in the Topological Semimetal Mo$_5$Si$_3$

By performing P doping on the Si sites in the topological semimetal Mo$_5$Si$_3$, we discover strong-coupling superconductivity in Mo$_5$Si$_{3-x}$P$_x$ (0.5 $\le$ $x$ $\le$ 2.0). Mo$_5$Si$_3$ crystallizes in the W$_5$Si$_3$-type structure with space group of $I4/mcm$ (No. 140), and is not a superconductor itself. Upon P doping, the lattice parameter $a$ decreases while $c$ increases monotonously. Bulk superconductivity is revealed in Mo$_5$Si$_{3-x}$P$_x$ (0.5 $\le$ $x$ $\le$ 2.0) from resistivity, magnetization, and heat capacity measurements. $T_c$ in Mo$_5$Si$_{1.5}$P$_{1.5}$ reaches as high as 10.8 K, setting a new record among the W$_5$Si$_3$-type superconductors. The upper and lower critical fields for Mo$_5$Si$_{1.5}$P$_{1.5}$ are 14.56 T and 105 mT, respectively. Moreover, Mo$_5$Si$_{1.5}$P$_{1.5}$ is found to be a fully gapped superconductor with strong electron-phonon coupling. First-principles calculations suggest that the enhancement of electron-phonon coupling is possibly due to the shift of the Fermi level, which is induced by electron doping. The calculations also reveal the nontrivial band topology in Mo$_5$Si$_3$. The $T_c$ and upper critical field in Mo$_5$Si$_{3-x}$P$_x$ are fairly high among pseudobinary compounds. Both of them are higher than those in NbTi, making future applications promising. Our results suggest that the W$_5$Si$_3$-type compounds are ideal platforms to search for new superconductors. By examinations of their band topologies, more candidates for topological superconductors can be expected in this structural family.

cond-mat.supr-con↗
arXiv2022

New Type of Quantum Oscillations Stemmed From the Strong Weyl Fermions - 4f Electrons Exchange Interaction

The interplay between magnetism and the topology of electronic band structure may generate new exotic quantum states. Here we report on a new type of quantum oscillations in the temperature dependent electrical resistivity and specific heat at a constant magnetic field in a polar magnetic Weyl semimetal (WSM) NdAlSi. These novel quantum phenomena arise from the destructive interference between quantum oscillations from the spin-split Fermi surfaces due to the strong Weyl fermions-4f electrons exchange interaction combined with Rashba-Dresselhaus (RD) and Zeeman effects. Our findings pave a way to explore unprecedented quantum phenomena in 4f-electron based magnetic semimetals.

cond-mat.mtrl-sci↗
arXiv2022

NdAlSi: a magnetic Weyl semimetal candidate with rich magnetic phases and atypical transport properties

Magnetic Weyl semimetals (MWSM) have attracted significant attention due to their intriguing physical properties and potential applications in spin-electronic devices. Here we report the characterization of NdAlSi including transport, magnetization, and heat capacity on single crystals, as well as band structure calculation. It is a newly proposed MWSM candidate which breaks both time-reversal and spacial inversion symmetries. A temperature-magnetic field phase diagram is experimentally established. Remarkably, on the angular magnetoresistance (AMR), a two-fold symmetric sharp peak instead of a smooth variation is observed in the field-induced ferrimagnetic phase. We argue that the tunability of both the topological and magnetic properties in NdAlSi is crucial for realizing such a behavior. Our results indicate that 4f-electron-based MWSM can provide a unique platform to explore new and intriguing quantum phenomena arising from the interaction between magnetism and topology.

cond-mat.mtrl-sci↗
arXiv2021

Large CaFeAsF single crystals of high quality grown by the oscillating temperature technique

The oscillating temperature technique was successfully employed to grow large and composition homogeneous CaFeAsF single crystals due to the reduction of nuclei number by temperature oscillation. The largest as-grown CaFeAsF single crystal was nearly 8 mm in lateral size and 0.27 mm in thickness which is far larger than previously reported in all dimensions. The full-width-at-half maximum (FWHM) of x-ray rocking curve is as small as 0.163° indicating the high quality of CaFeAsF single crystals.

cond-mat.supr-con↗
arXiv2021

Superconductivity with a Violation of Pauli Limit and Evidences for Multigap in $η$-Carbide type Ti$_4$Ir$_2$O

We report the synthesis, crystal structure, and superconductivity of Ti$_4$Ir$_2$O. The title compound crystallizes in an $η$-carbide type structure of the space group $Fd\overline{3}m$ (No. 227), with lattice parameters $a=b=c=11.6194(1)$ Å. The superconducting $T_c$ is found to be 5.1 $\sim$ 5.7 K. Most surprisingly, Ti$_4$Ir$_2$O hosts an upper critical field of 16.45 T, which is far beyond the Pauli paramagnetic limit. Strong coupled superconductivity with evidences for multigap is revealed by the measurements of heat capacity and upper critical field. First-principles calculations suggest that the density of states near the Fermi level originates from the hybridization of Ti-3$d$ and Ir-5$d$ orbitals, and the effect of spin-orbit coupling on the Fermi surfaces is prominent. Large values of the Wilson ratio ($R_W$ $\sim$ 3.9), the Kadowaki-Woods ratio ($A/γ^2$ $\sim$ 9.0 $\times$ 10$^{-6}$ $μΩ$ cm/(mJ mol$^{-1}$ K$^{-1}$)$^2$), and the Sommerfeld coefficient ($γ$ = 33.74 mJ mol$^{-1}$ K$^{-2}$) all suggest strong electron correlations (similar to heavy fermion systems) in Ti$_4$Ir$_2$O. The violation of Pauli limit is possibly due to a combination of strong-coupled superconductivity, large spin-orbit scattering, and electron correlation. With these intriguing behaviors, Ti$_4$Ir$_2$O serves as a candidate for unconventional superconductor.

cond-mat.supr-con↗
arXiv2020

Inelastic electron tunneling in 2H-Ta$_x$Nb$_{1-x}$Se$_2$ evidenced by scanning tunneling spectroscopy

We report a detailed study of tunneling spectra measured on 2H-Ta$_x$Nb$_{1-x}$Se$_2$ ($x=0\sim 0.1$) single crystals using a low-temperature scanning tunneling microscope. The prominent gap-like feature unintelligible for a long time was found to be accompanied by some "in-gap" fine structures. By investigating the second-derivative spectra and their temperature and magnetic field dependencies, we were able to prove that inelastic electron tunneling is the origin of these features and obtain the Eliashberg function of 2H-Ta$_x$Nb$_{1-x}$Se$_2$ at atomic scale, providing a potential way to study the local Eliashberg function and phonon spectra of the related transition-metal dichalcogenides.

cond-mat.mtrl-sci↗
arXiv2019

The upper critical field and its anisotropy in RbCr$_{3}$As$_{3}$

The temperature dependence of the upper critical field ($H_{c2}$) in RbCr$% _{3}$As$_{3}$ single crystals ($T_{c}\approx $ 7.3 K) has been determined by means of magnetoresistance measurements with temperature down to 0.35 K in static magnetic fields up to 38 T. The magnetic field was applied both for directions parallel ($H\parallel c $, $H_{c2}^{\parallel c}$) and perpendicular ($H\perp c$, $H_{c2}^{\perp c}$) to the Cr chains. The curves $H_{c2}^{\parallel c}(T)$ and $H_{c2}^{\perp c}(T)$ cross at $\sim $ 5.5 K. As a result, the anisotropy parameter $γ(T)=H_{c2}^{\perp c}/H_{c2}^{\parallel c}(T)$ increases from 0.5 near $T_{c}$ to 1.6 at low temperature. Fitting with the Werthamer-Helfand-Hohenberg (WHH) model yields zero-temperature critical fields of $μ_0H_{c2}^{\parallel c}(0)\approx $ 27.2 T and $μ_0H_{c2}^{\perp c}(0)\approx $ 43.4 T, both exceeding the BCS weak-coupling Pauli limit $μ_0H_{p}=1.84T_{c}=13.4$ T. The results indicate that the paramagnetic pair breaking effect is strong for $H \parallel c$ but absent for $H \perp c$, which was further confirmed by the angle dependent magnetoresistance and $H_{c2}$ measurements.

cond-mat.supr-con↗
arXiv2019

Multiband superconductivity and possible nodal gap in RbCr$_{3}$As$_{3}$ revealed by Andreev reflection and single-particle tunneling measurements

By measuring point-contact Andreev reflection (PCAR) spectra in the newly discovered chromium-based quasi-one-dimensional superconductor RbCr$_{3}$As$_{3}$, we find clear evidence of two superconducting components, i.e., one having a gap value of about 1.8 meV and another with a gap value of about 5 meV. Since the current injection may have components in both the direction parallel and perpendicular to the [(Cr$_{3}$As$_{3}$)$^{-}$]$_\infty$ chains in the PCAR measurements, it naturally explains the two-component feature observed in this multi-band superconductor. Detailed analysis shows that the larger gap may have an $s$-wave nature. We then carry out the single-particle tunneling measurements based on a scanning tunneling spectroscope by using the needle-like sample as the tip, and in this case the measured current is mainly parallel to the [(Cr$_{3}$As$_{3}$)$^{-}$]$_\infty$ chains. The single-particle tunneling spectra show only one gap feature with a gap value of about 1.8 meV. Fitting to the single particle tunneling spectra indicates that the gap should have a large anisotropy or even node(s). We argue that the absence of the larger gap may be related to the direction of the injecting current. Therefore, our combined experiments show the multiband superconductivity with one gap being nodal or highly anisotropic.

cond-mat.supr-con↗
arXiv2019

Superconductivity in Bi3O2S2Cl with Bi-Cl Planar Layers

A quaternary compound Bi3O2S2Cl, which consists of novel [BiS2Cl]2-layers, is reported. It adopts a layered structure of the space group I4/mmm (No. 139) with lattice parameters: a = 3.927(1) Å, c = 21.720(5) Å. In this compound, bismuth and chlorine atoms form an infinite planar layer, which is unique among the bismuth halides. Superconductivity is observed in both polycrystals and single crystals, and is significantly enhanced in the samples prepared with less sulfur or at higher temperatures. By tuning the content of sulfur, Bi3O2S2Cl can be converted from a semiconductor into a superconductor. The superconducting critical temperature ranges from 2.6 K to 3.5 K. Our discovery of the [BiS2Cl]2- layer opens another door in searching for the bismuth compounds with novel physical properties.

cond-mat.supr-con↗
arXiv2018

Superconductivity in LaPd2Bi2 with CaBe2Ge2-type structure

Here we report the synthesis and superconductivity of a novel ternary compound LaPd2Bi2. Shiny plate-like single crystals of LaPd2Bi2 were first synthesized by high-temperature solution method with PdBi flux. X-ray diffraction analysis indicates that LaPd2Bi2 belongs to the primitive tetragonal CaBe2Ge2-type structure with the space group P4/nmm (No. 129), and the refined lattice parameters are a = 4.717(2) Å, c = 9.957(3) Å. Electrical resistivity and magnetic susceptibility measurements reveal that LaPd2Bi2 undergoes a superconducting transition at 2.83 K and exhibits the characteristics of type-II superconductivity. The discovery of superconductivity in LaPd2Bi2 with CaBe2Ge2-type structure may help to further understand the possible relationship between the occurrence of superconductivity and the crystal structures in 122-type materials.

cond-mat.supr-con↗
arXiv2018

Superconductivity in novel quasi-one-dimensional ternary molybdenum pnictides Rb2Mo3As3 and Cs2Mo3As3

By replacing the alkali element in the newly discovered K2Mo3As3 superconductor, we successfully synthesized ternary molybdenum pnictides Rb2Mo3As3 and Cs2Mo3As3 through solid state reaction method. Powder X-ray diffraction analysis reveals the same quasi-one-dimensional (Q1D) hexagonal crystal structure and space group of P-6m2 (No. 187) as K2Mo3As3. The refined lattice parameters are a = 10.432 (1) Å, c = 4.4615 (6) Å for Rb2Mo3As3 and a = 10.7405 (6) Å, c = 4.4654 (5) Å for Cs2Mo3As3. Electrical resistivity and magnetic susceptibility characterizations exhibit the occurrence of superconductivity in both compounds with the onset Tc at 10.6 K and 11.5 K for Rb2Mo3As3 and Cs2Mo3As3 respectively, which exhibit weak negative chemical pressure effect in these A2Mo3As3 (A = K, Rb, Cs) superconductors contrary to the isostructural A2Cr3As3 superconductors. More interestingly, the Cs2Mo3As3 superconductor exhibits much higher upper critical field around 60 T at zero temperature. The discovery of these MoAs/CrAs-based superconductors provide a unique platform for the study of exotic superconductivity correlated with both 3d and 4d electrons in these Q1D compounds.

cond-mat.supr-con↗