Showing papers by "Zhu-An Xu published in 2015"

Unconventional superconductivity in quasi-one-dimensional Rb 2 Cr 3 As 3

Abstract: Following the authors' own discovery of superconductivity in quasi-one-dimensional K${}_{2}$Cr${}_{3}$As${}_{3}$, the analogous compound Rb${}_{2}$Cr${}_{3}$As${}_{3}$ is synthesized for the first time in polycrystalline form. Bulk superconductivity emerges at 4.8 K. Further work is needed to verify the nature of the superconducting state.

Superconductivity in Quasi-One-Dimensional K 2 Cr 3 As 3 with Significant Electron Correlations

Abstract: Superconductivity, the absence of electrical resistance, is rare in quasi-one-dimensional materials Researchers show that bulk superconductivity emerges at 61 K and ambient pressure in a quasi-one-dimensional chromium arsenide

Superconductivity in quasi-one-dimensional Cs 2 Cr 3 As 3 with large interchain distance

Abstract: 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 the unconventional SC. Our recent observation of SC at 6.1 K in correlated electron material K2Cr3As3 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 the 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 a common unconventional SC in the Q1D Cr-based material.

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

Abstract: 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.

Observation of Fermi Arcs in non-Centrosymmetric Weyl Semi-metal Candidate NbP

Abstract: We report the surface electronic structure of niobium phosphide NbP single crystal on (001) surface by vacuum ultraviolet angle-resolved photoemission spectroscopy. Combining with our first principle calculations, we identify the existence of the Fermi arcs originated from topological surface states. Furthermore, the surface states exhibit circular dichroism pattern, which may correlate with its non-trivial spin texture. Our results provide critical evidence for the existence of the Weyl Fermions in NbP, which lays the foundation for further research.

Cluster spin-glass ground state in quasi-one-dimensional KCr 3 As 3

Abstract: We report structural and physical properties of a new quasi-one-dimensional Cr-based compound, ${\mathrm{KCr}}_{3}{\mathrm{As}}_{3}$, which is prepared by potassium deintercalation from the superconductive ${\mathrm{K}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{3}$. ${\mathrm{KCr}}_{3}{\mathrm{As}}_{3}$ adopts the ${\mathrm{TlFe}}_{3}{\mathrm{Te}}_{3}$-type structure with space group $P{6}_{3}/m$ (No. 176). The high-temperature magnetic susceptibility obeys the Curie-Weiss law with an effective magnetic moment of 0.68 ${\ensuremath{\mu}}_{\mathrm{B}}$/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 $\ensuremath{\sim}5$ K.

Coexistence of superconductivity and ferromagnetism in Sr 0.5 Ce 0.5 FBiS 2

Abstract: Through the combination of x-ray diffraction, electrical transport, magnetic susceptibility, and heat capacity measurements, we studied the effect of Ce doping in the newly discovered ${\mathrm{SrFBiS}}_{2}$ system. It is found that ${\mathrm{Sr}}_{0.5}{\mathrm{Ce}}_{0.5}{\mathrm{FBiS}}_{2}$ undergoes a second-order transition below $\ensuremath{\sim}7.5$ K, followed by a superconducting transition with the critical temperature ${T}_{c}\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}2.8$ K. Our transport, specific heat, and dc-magnetization results suggest the presence of bulk ferromagnetic correlation of Ce ions below 7.5 K that coexist with superconductivity when the temperature is further lowered below 2.8 K.

Chiral anomaly induced negative magnetoresistance in topological Weyl semimetal NbAs

Abstract: In this paper, we report the intercone transport of Weyl fermions in NbAs with external magnetic field in parallel to electric field, a quantum phenomenon known as the Adler-Bell-Jackiw anomaly. Surprisingly, the resulting negative magnetoresistance (MR) in NbAs shows significant difference from NbP. The observed low-field positive MR dip, which is missing in NbP at low temperatures, indicates that the spin-orbital coupling (SOC) is significantly stronger in NbAs than in the former. The results imply that the contribution of arsenic to SOC in TaAs and NbAs is not negligible.

Synthesis, crystal structure and physical properties of quasi-one-dimensional ACr 3 As 3 (A = Rb, Cs)

Abstract: Recently, new Cr-based superconductors, A2Cr3As3 (A = K, Rb, Cs), have gained intense interest because of their one-dimensional crystal structures and electron correlations. Here we report the crystal structure and physical properties of two related materials ACr3As3 (A = Rb, Cs) which are synthesized via a soft-chemical A+ deintercalation in A2Cr3As3. The new compounds remain one-dimensional (Cr3As3)∞ 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 5–6 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 A2Cr3As3.

Coexistence of ferromagnetism and superconductivity in iron based pnictides: a time resolved magnetooptical study

Abstract: Ferromagnetism and superconductivity are antagonistic phenomena. Their coexistence implies either a modulated ferromagnetic order parameter on a lengthscale shorter than the superconducting coherence length or a weak exchange coupling between the itinerant superconducting electrons and the localized ordered spins. In some iron based pnictide superconductors the coexistence of ferromagnetism and superconductivity has been clearly demonstrated. The nature of the coexistence, however, remains elusive since no clear understanding of the spin structure in the superconducting state has been reached and the reports on the coupling strength are controversial. We show, by a direct optical pump-probe experiment, that the coupling is weak, since the transfer of the excess energy from the itinerant electrons to ordered localized spins is much slower than the electron-phonon relaxation, implying the coexistence without the short-lengthscale ferromagnetic order parameter modulation. Remarkably, the polarization analysis of the coherently excited spin wave response points towards a simple ferromagnetic ordering of spins with two distinct types of ferromagnetic domains.

Physical properties and electronic structure of Sr 2 Cr 3 As 2 O 2 containing CrO 2 and Cr 2 As 2 square-planar lattices

Abstract: We report the physical properties and electronic structure calculations of a layered chromium oxypnictide, Sr$_2$Cr$_3$As$_2$O$_2$, which crystallizes in a Sr$_2$Mn$_3$As$_2$O$_2$-type structure containing both CrO$_2$ planes and Cr$_2$As$_2$ layers. The newly synthesized material exhibits a metallic conduction with a dominant electron-magnon scattering. Magnetic and specific-heat measurements indicate at least two intrinsic magnetic transitions below room temperature. One is an antiferromagnetic transition at 291 K, probably associated with a spin ordering in the Cr$_2$As$_2$ layers. Another transition is broad, occurring at around 38 K, and possibly due to a short-range spin order in the CrO$_2$ planes. Our first-principles calculations indicate predominant two-dimensional antiferromagnetic exchange couplings, and suggest a KG-type (i.e. K$_2$NiF$_4$ type for CrO$_2$ planes and G type for Cr$_2$As$_2$ layers) magnetic structure, with reduced moments for both Cr sublattices. The corresponding electronic states near the Fermi energy are mostly contributed from Cr-3$d$ orbitals which weakly (modestly) hybridize with the O-2$p$ (As-4$p$) orbitals in the CrO$_2$ (Cr$_2$As$_2$) layers. The bare bandstructure density of states at the Fermi level is only $\sim$1/4 of the experimental value derived from the low-temperature specific-heat data, consistent with the remarkable electron-magnon coupling. The title compound is argued to be a possible candidate to host superconductivity.

Dominant Spin-Triplet Superconductivity Revealed by the Upper-Critical-Field Measurements in K$_{2}$Cr$_{3}$As$_{3}$

Abstract: The upper critical magnetic field $H_{\mathrm{c2}}$ of a superconductor measures the strength of superconductivity in response to external magnetic fields, which gives clues to the superconducting Cooper pairing. In this paper, we report measurements of $H_{\mathrm{c2}}$ as functions of temperature $T$, polar angle $\theta$ and azimuthal angle $\phi$ of the applied magnetic field direction, for a newly discovered superconductor K$_{2}$Cr$_{3}$As$_{3}$ with a quasi-one-dimensional and non-centrosymmetric crystal structure. We demonstrate that the anisotropy reversal in $H_{\mathrm{c2}}(T)$ reported previously comes from a fully anisotropic Pauli-limiting effect where there is no paramagnetic pair breaking for $\mathbf{H}\bot c$. The $H_{\mathrm{c2}}(\theta)$ data reveal an intrinsic uniaxially effective-mass anisotropy ($m_{\bot}/m_{\parallel}\sim$ 2.0) down to low temperatures. The orbitally limited fields are extrapolated to be 372 kOe and 510 kOe for $\mathbf{H}\parallel c$ and $\mathbf{H}\bot c$ respectively at zero temperature, which exceed the Pauli limit by a factor of 3.4 and 4.6. More interestingly, the in-plane $H_{\mathrm{c2}}(\phi)$ profile shows a unique threefold modulation, which breaks the time-reversal symmetry. The above $H_{\mathrm{c2}}(\theta, \phi, T)$ behaviors cannot be understood unless a spin-triplet superconductivity with odd parity is present in K$_{2}$Cr$_{3}$As$_{3}$.

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

Abstract: 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$.

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

Abstract: 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.

Coexistence of superconductivity and complex 4 f magnetism in Eu0.5Ce0.5BiS2F.

Abstract: EuBiS2F 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 BiS2 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-6p orbitals that are responsible for the 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.

Observation of Fermi Arcs in non-Centrosymmetric Weyl Semi-metal Candidate NbP

Abstract: We report the surface electronic structure of niobium phosphide NbP single crystal on (001) surface by vacuum ultraviolet angle-resolved photoemission spectroscopy. Combining with our first principle calculations, we identify the existence of the Fermi arcs originated from topological surface states. Furthermore, the surface states exhibit circular dichroism pattern, which may correlate with its non-trivial spin texture. Our results provide critical evidence for the existence of the Weyl Fermions in NbP, which lays the foundation for further investigations.

Observation of Negative Magnetoresistance and nontrivial $\pi$ Berrys phase in 3D Weyl semi-metal NbAs

Abstract: We report the electric transport properties of NbAs, which is a Weyl semimetal candidate proposed by recent theoretical calculations and confirmed by recent angle-resolved photoemission spectroscopy (ARPES) data. We detected the long-anticipated negative magneto-resistance generated by the chiral anomaly in NbAs. Clear Shubnikov de Haas (SdH) oscillations have been detected starting from very weak magnetic field. Analysis of the SdH peaks gives the Berry phase accumulated along the cyclotron orbits to be $\pi$, indicating the existence of Weyl points.

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

Abstract: 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 $\kappa_{\text{GL}}=$ 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.

Vortex crossing and trapping in doubly connected mesoscopic loops of a single-crystal type-II superconductor

Abstract: Numerical calculations on a mesoscopic ring of a type-II superconductor in the London limit suggest that an Abrikosov vortex can be trapped in such a structure above a critical magnetic field and generate a phase shift in the magnetoresistance oscillations. We prepared submicron-sized superconducting loops of single-crystal, type-II superconductor $\mathrm{NbSe}_{2}$ and measured magnetoresistance oscillations resulting from vortices crossing the loops. The free-energy barrier for vortex crossing determines the crossing rate and is periodically modulated by the external magnetic flux threading the loop. We demonstrated experimentally that the crossing of vortices can be directed at a pair of constrictions in the loop, leading to more pronounced magnetoresistance oscillations than those in a uniform ring. The vortex trapping in both a simple ring and a ring featuring two constrictions was found to result in a phase shift in the magnetoresistance oscillations as predicted in the numerical calculations. The controlled crossing and trapping of vortices demonstrated in our $\mathrm{NbSe}_{2}$ devices provide a starting point for the manipulation of individual Abrikosov vortices, which is useful for future technologies.

Multiband superconductivity in Ta4Pd3Te16 with anisotropic gap structure.

Abstract: We carried out measurements of the magnetoresistance, magnetic susceptibility and specific heat on crystals of the low-dimensional transition metal telluride Ta4Pd3Te16. Our results indicate that Ta4Pd3Te16 is an anisotropic type-II superconductor in the clean limit with the extracted Ginzburg?Landau parameter 84. The upper critical field Hc2(T) shows an anomalous temperature dependence at low temperatures and the anisotropy of Hc2(T) is strongly T-dependent, both of which indicate a multiband scenario. The electronic specific heat (T) can be consistently described by a two-gap (s???+???d waves) model from the base temperature up to . Our results suggest nodal and multiband superconductivity in Ta4Pd3Te16.

Local noncentrosymmetricity and possible spin-momentum locking in Sr$_3$Ru$_2$O$_7$

Abstract: Strong spin-orbital coupling (SOC) was found previously to lead to dramatic effects in quantum materials, such as those found in topological insulators. It was shown theoretically that local noncentrosymmetricity resulting from the rotation of RuO$_6$ octahedral in Sr$_3$Ru$_2$O$_7$ will also give rise to an effective SOC\cite{SocSr327,MicroscopicnematicSr327}. In the presence of a magnetic field applied along a specific in-plane direction, the Fermi surface was predicted to undergo a reconstruction. Here we report results of our in-plane magnetoresistivity and magnetothermopower measurements on single crystals of Sr$_3$Ru$_2$O$_7$ with an electrical or a thermal current applied along specific crystalline directions and a magnetic field rotating in the $ab$ plane (Fig. 1a), showing a minimal value for field directions predicted by the local noncentrosymmetricity theory. Furthermore, the thermopower, and therefore, the electron entropy, were found to be suppressed as the field was applied perpendicular to the thermal current, which suggests that the spin and the momentum in Sr$_3$Ru$_2$O$_7$ are locked over substantial parts of the Fermi surface, likely originating from local noncentrosymmetricity as well.