arXiv subjects

Gen-Fu Chen

Publications and source records attributed to Gen-Fu Chen.

At least 19 recordsLinked to original sources

arXiv2024

Non-equilibrium States and Interactions in the Topological Insulator and Topological Crystalline Insulator Phases of NaCd4As3

Topological materials are of great interest because they can support metallic edge or surface states that are robust against perturbations, with the potential for technological applications. Here we experimentally explore the light-induced non-equilibrium properties of two distinct topological phases in NaCd4As3: a topological crystalline insulator (TCI) phase and a topological insulator (TI) phase. This material has surface states that are protected by mirror symmetry in the TCI phase at room temperature, while it undergoes a structural phase transition to a TI phase below 200 K. After exciting the TI phase by an ultrafast laser pulse, we observe a leading band edge shift of >150 meV, that slowly builds up and reaches a maximum after ~0.6 ps, and that persists for ~8 ps. The slow rise time of the excited electron population and electron temperature suggests that the electronic and structural orders are strongly coupled in this TI phase. It also suggests that the directly excited electronic states and the probed electronic states are weakly coupled. Both couplings are likely due to a partial relaxation of the lattice distortion, which is known to be associated with the TI phase. In contrast, no distinct excited state is observed in the TCI phase immediately or after photoexcitation, which we attribute to the low density of states and phase space available near the Fermi level. Our results show how ultrafast laser excitation can reveal the distinct excited states and interactions in phase-rich topological materials.

cond-mat.mtrl-sci↗

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 $\xi$ and enhancement of the penetration depth $\lambda$ result in nearly factor of three increase of the Ginzburg-Landau parameter $\kappa=\lambda/\xi$ (from $\kappa\simeq 25$ for $x\lesssim 1.2$ to $\kappa\simeq 70$ 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 \AA and $c$ = 5.30031 \AA. 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

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↗

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↗

arXiv2021

Superconductivity with a Violation of Pauli Limit and Evidences for Multigap in $\eta$-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 $\eta$-carbide type structure of the space group $Fd\overline{3}m$ (No. 227), with lattice parameters $a=b=c=11.6194(1)$ \AA. 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 ($A/\gamma^2$ $\sim$ 9.0 $\times$ 10$^{-6}$ $\mu\Omega$ cm/(mJ mol$^{-1}$ K$^{-1}$)$^2$), and the Sommerfeld coefficient ($\gamma$ = 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

Spontaneous Formation of a Superconductor-Topological Insulator-Normal Metal Layered Heterostructure

The discovery of graphene has spurred vigorous investigation of 2D materials, revealing a wide range of extraordinary properties and functionalities. 2D heterostructural materials have recently been fabricated by assembling isolated planes layer-by-layer in a desired sequence. Unusual properties and novel physical phenomena have been unveiled in such layered heterostructures. For example, Hofstadter's butterfly, an intriguing pattern of the energy states of Bloch electrons, was predicted several decades ago to be observable only under unfeasibly strong magnetic fields in conventional materials. But it has been observed recently under current experimental conditions in graphene/BN layered heterostructures, one of the outstanding new kinds of 2D materials. Moreover, another amazing physics phenomenon, Majorana fermions was predicted to exist in heterostructural systems consisting of a superconductor (SC) and a topological insulator (TI) Journal.

cond-mat.mes-hall↗

arXiv2020

Experimental observations indicating the topological nature of the edge states on HfTe5

The topological edge states of two-dimensional topological insulators with large energy gap furnish ideal conduction channels for dissipationless current transport. Transition metal tellurides XTe5 (X=Zr, Hf) are theoretically predicted to be large-gap two-dimensional topological insulators and the experimental observations of their bulk insulating gap and in-gap edge states have been reported, but the topological nature of these edge states still remains to be further elucidated. Here, we report our low temperature scanning tunneling microscopy/spectroscopy study on single crystals of HfTe5. We demonstrate a full energy gap of ~80 meV near the Fermi level on the surface monolayer of HfTe5 and that such insulating energy gap gets filled with finite energy states when measured at the monolayer step edges. Remarkably, such states are absent at the edges of a narrow monolayer strip of one-unit-cell in width but persist at both step edges of a unit-cell wide monolayer groove. These experimental observations strongly indicate that the edge states of HfTe5 monolayers are not trivially caused by translational symmetry breaking, instead they are topological in nature protected by the 2D nontrivial bulk properties.

cond-mat.mtrl-sci↗

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

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) {\AA}, c = 21.720(5) {\AA}. 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) {\AA}, c = 9.957(3) {\AA}. 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) {\AA}, c = 4.4615 (6) {\AA} for Rb2Mo3As3 and a = 10.7405 (6) {\AA}, c = 4.4654 (5) {\AA} 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↗

arXiv2018

Superconductivity at 10.4 K in a novel quasi-one-dimensional ternary molybdenum pnictide K2Mo3As3

Here we report the discovery of the first ternary molybdenum pnictide based superconductor K2Mo3As3. Polycrystalline samples were synthesized by the conventional solid state reaction method. X-ray diffraction analysis reveals a quasi-one-dimensional hexagonal crystal structure with (Mo3As3)2- linear chains separated by K+ ions, similar as previously reported K2Cr3As3, with the space group of P-6m2 (No. 187) and the refined lattice parameters a = 10.145(5) {\AA} and c = 4.453(8) {\AA}. Electrical resistivity, magnetic susceptibility, and heat capacity measurements exhibit bulk superconductivity with the onset Tc at 10.4 K in K2Mo3As3 which is higher than the isostructural Cr-based superconductors. Being the same group VIB transition elements and with similar structural motifs, these Cr and Mo based superconductors may share some common underlying origins for the occurrence of superconductivity and need more investigations to uncover the electron pairing within a quasi-one-dimensional chain structure.

cond-mat.supr-con↗

arXiv2018

Ion-exchange synthesis and superconductivity at 8.6 K of Na2Cr3As3 with quasi-one-dimensional crystal structure

A new Cr-based quasi-one-dimensional superconductor Na2Cr3As3 was synthesized by an ion-exchange method in sodium naphthalenide solution. The crystals are thread-like and the structure was analyzed by X-ray diffraction with a noncentrosymmetric hexagonal space group P-6m2 (No. 187), in which the (Cr3As3)2- linear chains are separated by Na+ ions, and the refined lattice parameters are a = 9.239(2) {\AA} and c = 4.209(6) {\AA}. The measurements for electrical resistivity, magnetic susceptibility, and heat capacity reveal a superconducting transition with unconventional characteristic at the Tc of 8.6 K, which exceeds that of all previously reported Cr-based superconductors.

cond-mat.supr-con↗

arXiv2017

Superconductivity at 7.3 K in the 133-type Cr-based RbCr3As3 single crystals

Here we report the preparation and superconductivity of the 133-type Cr-based quasi-one-dimensional (Q1D) RbCr3As3 single crystals. The samples were prepared by the deintercalation of Rb+ ions from the 233-type Rb2Cr3As3 crystals which were grown from a high-temperature solution growth method. The RbCr3As3 compound crystallizes in a centrosymmetric structure with the space group of P63/m (No. 176) different with its non-centrosymmetric Rb2Cr3As3 superconducting precursor, and the refined lattice parameters are a = 9.373(3) {\AA} and c = 4.203(7) {\AA}. Electrical resistivity and magnetic susceptibility characterizations reveal the occurrence of superconductivity with an interestingly higher onset Tc of 7.3 K than other Cr-based superconductors, and a high upper critical field Hc2(0) near 70 T in this 133-type RbCr3As3 crystals.

cond-mat.supr-con↗

arXiv2017

Revisiting the electron-doped SmFeAsO: enhanced superconductivity up to 58.6 K by Th and F codoping

In the iron-based high-Tc bulk superconductors, Tc above 50K was only observed in the electron-doped 1111-type compounds. Here we revisit the electron-doped SmFeAsO polycrystals to make a further investigation for the highest T-c in these materials. To introduce more electron carriers and less crystal lattice distortions, we study the Th and F codoping effects into the Sm-O layers with heavy electron doping. Dozens of Sm1-x Th-x FeAsO1-y F-y samples are synthesized through the solid state reaction method, and these samples are carefully characterized by the structural, resistive, and magnetic measurements. We find that the codoping of Th and F clearly enhances the superconducting T-c more than the Th or F single-doped samples, with the highest record T-c up to 58.6K when x= 0.2 and y= 0.225. Further element doping causes more impurities and lattice distortions in the samples with a weakened superconductivity.

cond-mat.supr-con↗