arXiv subjects

Chun-Mu Feng

Publications and source records attributed to Chun-Mu Feng.

14 recordsLinked to original sources

arXiv2018

Peculiar Phase Diagram with Isolated Superconducting Regions in ThFeAsN$_{1-x}$O$_x$

ThFeAsN$_{1-x}$O$_x$ ($0\leq x\leq0.6$) system with heavy electron doping has been studied by the measurements of X-ray diffraction, electrical resistivity, magnetic susceptibility and specific heat. The non-doped compound exhibits superconductivity at $T_\mathrm{c}^\mathrm{onset}=30$ K, which is possibly due to an internal uniaxial chemical pressure that is manifested by the extremely small value of As height with respect to the Fe plane. With the oxygen substitution, the $T_\mathrm{c}$ value decreases rapidly to below 2 K for $0.1\leq x\leq0.2$, and surprisingly, superconductivity re-appears in the range of $0.25\leq x\leq0.5$ with a maximum $T_\mathrm{c}^\mathrm{onset}$ of 17.5 K at $x=0.3$. For the normal-state resistivity, while the samples in intermediate non-superconducting interval exhibit Fermi liquid behavior, those in other regions show a non-Fermi-liquid behavior. The specific heat jump for the superconducting sample of $x=0.4$ is $ΔC/(γT_\mathrm{c})=0.89$, which is discussed in terms of anisotropic superconducting gap. The peculiar phase diagram in ThFeAsN$_{1-x}$O$_x$ presents additional ingredients for understanding the superconducting mechanism in iron-based superconductors.

cond-mat.supr-con↗

arXiv2018

RbEu(Fe$_{1-x}$Ni$_x$)$_4$As$_4$: From a ferromagnetic superconductor to a superconducting ferromagnet

The intrinsically hole-doped RbEuFe$_4$As$_4$ exhibits bulk superconductivity at $T_{\mathrm{sc}}=36.5$ K and ferromagnetic ordering in the Eu sublattice at $T_\mathrm{m}=15$ K. Here we present a hole-compensation study by introducing extra itinerant electrons via a Ni substitution in the ferromagnetic superconductor RbEuFe$_4$As$_4$ with $T_{\mathrm{sc}}>T_{\mathrm{m}}$. With the Ni doping, $T_{\mathrm{sc}}$ decreases rapidly, and the Eu-spin ferromagnetism and its $T_{\mathrm{m}}$ remain unchanged. Consequently, the system RbEu(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ transforms into a superconducting ferromagnet with $T_{\mathrm{m}}>T_{\mathrm{sc}}$ for $0.07\leq x\leq0.08$. The occurrence of superconducting ferromagnets is attributed to the decoupling between Eu$^{2+}$ spins and superconducting Cooper pairs. The superconducting and magnetic phase diagram is established, which additionally includes a recovered yet suppressed spin-density-wave state.

cond-mat.supr-con↗

arXiv2016

Superconductivity at 35 K by self doping in RbGd$_2$Fe$_4$As$_4$O$_2$

We report synthesis, crystal structure and physical properties of a novel quinary compound RbGd$_2$Fe$_4$As$_4$O$_2$. The new iron oxyarsenide is isostructural to the fluo-arsenide KCa$_2$Fe$_4$As$_4$F$_2$, both of which contain separate double Fe$_2$As$_2$ layers that are self hole-doped in the stoichiometric composition. Bulk superconductivity at $T_\mathrm{c}$ = 35 K is demonstrated by the measurements of electrical resistivity, dc magnetic susceptibility and heat capacity. An exceptionally high value of the initial slope of the upper critical field ($μ_0$d$H_{\mathrm{c2}}$/d$T|_{T_\mathrm{c}}$ = 16.5 T/K) is measured for the polycrystalline sample.

cond-mat.supr-con↗

arXiv2016

Superconductivity in KCa$_2$Fe$_4$As$_4$F$_2$ with Separate Double Fe$_2$As$_2$ Layers

We report synthesis, crystal structure and physical properties of a quinary iron-arsenide fluoride KCa$_2$Fe$_4$As$_4$F$_2$. The new compound crystallizes in a body-centered tetragonal lattice (with space group $I4/mmm$, $a$ = 3.8684(2) Å, c = 31.007(1) Å, and $Z$ = 2), which contains double Fe$_2$As$_2$ conducting layers separated by insulating Ca$_2$F$_2$ layers. Our measurements of electrical resistivity, dc magnetic susceptibility and heat capacity demonstrate bulk superconductivity at 33 K in KCa$_2$Fe$_4$As$_4$F$_2$.

cond-mat.supr-con↗

arXiv2016

Superconductivity and Ferromagnetism in Hole-Doped RbEuFe$_4$As$_4$

We discover a robust coexistence of superconductivity and ferromagnetism in an iron arsenide RbEuFe$_4$As$_4$. The new material crystallizes in an intergrowth structure of RbFe$_2$As$_2$ and EuFe$_2$As$_2$, such that the Eu sublattice turns out to be primitive instead of being body-centered in EuFe$_2$As$_2$. The FeAs layers, featured by asymmetric As coordinations, are hole doped due to charge homogenization. Our combined measurements of electrical transport, magnetization and heat capacity unambiguously and consistently indicate bulk superconductivity at 36.5 K in the FeAs layers and ferromagnetism at 15 K in the Eu sublattice. Interestingly, the Eu-spin ferromagnetic ordering belongs to a rare third-order transition, according to the Ehrenfest classification of phase transition. We also identify an additional anomaly at $\sim$ 5 K, which is possibly associated with the interplay between superconductivity and ferromagnetism.

cond-mat.supr-con↗

arXiv2015

Synthesis, crystal structure and physical properties of quasi-one-dimensional ACr$_3$As$_3$ (A = Rb, Cs)

Recently, new Cr-based superconductors, A$_2$Cr$_3$As$_3$ (A = K, Rb, Cs), have gained a strong interest because of their one-dimensional crystal structures and electron correlations. Here we report the crystal structure and physical properties of two related materials ACr$_3$As$_3$ (A = Rb, Cs) which are synthesized via a soft-chemical A+ deintercalation in A$_2$Cr$_3$As$_3$. The new compounds remain one-dimensional (Cr$_3$As$_3$)$_{\infty}$ linear chains, and the interchain distance can be tuned by the incorporation of the alkali-metal cations with different sizes. The physical-property measurements indicate a local-moment behavior at high temperatures, and the moments freeze into a cluster spin-glass state below 5$\sim$6 K. No superconductivity was observed in both materials. We also found that, with increasing the interchain distance, the Cr effective moments increase monotonically, accompanied with the enhancement of semi-conductivity. Our results shed light on the understanding of occurrence of superconductivity in A$_2$Cr$_3$As$_3$.

cond-mat.str-el↗

arXiv2015

Cluster spin-glass ground state in quasi-one-dimensional KCr$_{3}$As$_{3}$

We report structural and physical properties of a new quasi-one-dimensional Cr-based compound, KCr$_{3}$As$_{3}$, which is prepared by potassium deintercalation from the superconductive K$_{2}$Cr$_{3}$As$_{3}$. KCr$_{3}$As$_{3}$ adopts the TlFe$_{3}$Te$_{3}$-type structure with space group $P6_{3}$/$m$ (No. 176). The high-temperature magnetic susceptibility obeys the Curie-Weiss law with an effective magnetic moment of 0.68 $μ_{\mathrm{B}}$/Cr. Below 56 K the susceptibility deviates from the high-temperature Curie-Weiss behavior, coinciding with the rapid increase in resistivity, which suggests formation of spin clusters. The short-range spin correlations are also supported by the specific-heat data. The title material does not exhibit bulk superconductivity; instead, it shows a cluster spin-glass state below $\sim$ 5 K.

cond-mat.supr-con↗

arXiv2015

Coexistence of superconductivity and complex 4$f$ magnetism in Eu$_{0.5}$Ce$_{0.5}$BiS$_{2}$F

EuBiS$_{2}$F is a self-doped superconductor due to the mixed valence of Eu. Here we report that, with the Ce substitution for Eu by 50 at.\%, the material exhibits ferromagnetic ordering at 8 K for the Ce-4$f$ moment, superconductivity at 2.2 K in the BiS$_2$ layers, and possibly antiferromagnetic ordering at 2.1 K for the Eu-4$f$ spins. The Eu valence is essentially divalent with the Ce incorporation. We tentatively interpret the coexistence of ferromagnetism and superconductivity by considering different Bi-6$p$ orbitals that are responsible for superconductivity itself and for mediating the ferromagnetic interaction, respectively. We argue that the antiferromagnetic ordering of the Eu-4$f$ spins is most likely due to a magnetic dipole-dipole interaction.

cond-mat.supr-con↗

arXiv2015

Unconventional superconductivity in quasi-one-dimensional Rb$_2$Cr$_3$As$_3$

Following the discovery of superconductivity in quasi-one-dimensional K$_2$Cr$_3$As$_3$ containing [(Cr$_3$As$_3$)$^{2-}$]$_{\infty}$ chains [J. K. Bao et al., arXiv: 1412.0067 (2014)], we succeeded in synthesizing an analogous compound, Rb$_2$Cr$_3$As$_3$, which also crystallizes in a hexagonal lattice. The replacement of K by Rb results in an expansion of $a$ axis by 3\%, indicating a weaker interchain coupling in Rb$_2$Cr$_3$As$_3$. Bulk superconductivity emerges at 4.8 K, above which the normal-state resistivity shows a linear temperature dependence up to 35 K. The estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit by a factor of two. Furthermore, the electronic specific-heat coefficient extrapolated to zero temperature in the mixed state increases with $\sqrt{H}$, suggesting existence of nodes in the superconducting energy gap. Hence Rb$_2$Cr$_3$As$_3$ manifests itself as another example of unconventional superconductor in the Cr$_3$As$_3$-chain based system.

cond-mat.supr-con↗

arXiv2015

Superconductivity in quasi-one-dimensional K$_2$Cr$_3$As$_3$ with significant electron correlations

We report the discovery of bulk superconductivity (SC) at 6.1 K in a quasi-one-dimensional (Q1D) chromium pnictide K$_2$Cr$_3$As$_3$ which contains [(Cr$_3$As$_3$)$^{2-}$]$_{\infty}$ double-walled subnano-tubes with face-sharing Cr$_{6/2}$ (As$_{6/2}$) octahedron linear chains in the inner (outer) wall. The material has a large electronic specific-heat coefficient of 70$\sim$75 mJ K$^{-2}$ mol$^{-1}$, indicating significantly strong electron correlations. Signature of non-Fermi liquid behavior is shown by the linear temperature dependence of resistivity in a broad temperature range from 7 to 300 K. Unconventional SC is preliminarily manifested by the estimated upper critical field exceeding the Pauli limit by a factor of three to four. The title compound represents a rare example that possibly unconventional SC emerges in a Q1D system with strong electron correlations.

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arXiv2015

Multiband superconductivity in Ta$_4$Pd$_3$Te$_{16}$ with anisotropic gap structure

We carried out the measurements of magnetoresistance, magnetic susceptibility and specific heat on crystals of the low-dimensional transition metal telluride Ta$_4$Pd$_3$Te$_{16}$. Our results indicate that Ta$_4$Pd$_3$Te$_{16}$ is an anisotropic type-II superconductor with the extracted Ginzburg-Landau parameter $κ_{\text{GL}}=$ 84. The upper critical field $H_{c2}$($T$) shows a linear dependence at low temperature and the anisotropy of $H_{c2}$($T$) is strongly $T$-dependent, both of which indicate a multiband scenario. A detailed analysis reveals that the electronic specific heat $C_{\text{el}}$($T$) can be consistently described by a two-gap ($s$+$d$ waves) model from the base temperature $T/T_c\sim$ 0.12 up to $T_c$. Our data suggests multiband superconductivity in Ta$_4$Pd$_3$Te$_{16}$ with anisotropic gap structure.

cond-mat.supr-con↗

arXiv2015

Superconductivity in quasi-one-dimensional Cs2Cr3As3 with large interchain distance

Since the discovery of high-temperature superconductivity (SC) in quasi-two-dimensional copper oxides, a few layered compounds, which bear similarities to the cuprates, have also been found to host unconventional SC. Our recent observation of SC at 6.1 K in correlated electron material K2Cr3As3 (J. K. Bao et al., arXiv: 1412.0067) represents an obviously different paradigm, primarily because of its quasi-one-dimensional (Q1D) nature. The new material is structurally featured by the (Cr3As3)2- double-walled subnano-tubes composed of face-sharing Cr6/2 (As6/2) octahedron linear chains, which are well separated by columns of K+ counterions. Later, an isostructural superconducting Rb2Cr3As3 was synthesized, thus forming a new superconducting family. Here we report the third member, Cs2Cr3As3, which possesses the largest interchain distance. SC appears below 2.2 K. Similar to the former two sister compounds, Cs2Cr3As3 exhibits a non-Fermi liquid behavior with a linear temperature dependence of resistivity in the normal state, and a high upper critical field beyond the Pauli limit as well, suggesting common unconventional SC in the Q1D Cr-based material.

cond-mat.supr-con↗

arXiv2014

Superconductivity in a Layered Ta4Pd3Te16 with PdTe2 Chains

Superconductivity in low-dimensional compounds has long attracted much interest. Here we report superconductivity in a low-dimensional ternary telluride Ta4Pd3Te16 in which the repeating layers contain edge-sharing octahedrally-coordinated PdTe2 chains along the crystallographic b axis. Measurements of electrical resistivity, magnetic susceptibility and specific heat on the Ta4Pd3Te16 crystals, grown via a self-flux method, consistently demonstrate bulk superconductivity at 4.6 K. Further analyses of the data indicate significant electron-electron interaction, which allows electronic Cooper pairing in the present system.

cond-mat.supr-con↗

arXiv2014

Charge-density wave, superconductivity and $f$-electron valence instability in EuBiS$_2$F

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states which have received sustained interest for decades. Here we report that, in a layered europium bismuth sulfofluoride, EuBiS$_2$F, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane Bi-6$p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by ~ 50 times, owing to the significant hybridizations between Eu-4$f$ and Bi-6$p$ electrons, as verified by band-structure calculations. Thus, EuBiS$_2$F manifests itself as an unprecedented material that simultaneously accommodates SC, CDW and $f$-electron valence instability.

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