Search

A new Ba2CuO4-y superconductor with critical temperature (Tc) exceeding 70 K was discovered. The X-ray absorption measurement gives evidence that this cuprate resembles La2CuO4 but is doped with a fairly large amount of holes, while in contrast to the so far known hole-doped high-Tc cuprates, the new cuprate possesses a much shorter local apical oxygen distance as well as much expanded in-plane Cu–O bond, leading to unprecedented compressed local octahedron. In compressed local octahedron, the Cu3d3z2–r2 orbital level will be lifted above the Cu3dx2-y2 orbital level with more three-dimensional features, implying that pairing symmetry may carry admixtures from more than one gap, suggesting that Ba2CuO4-y composed of alkaline earth copper oxides that are the essential elements to form cuprate superconductors may belong to a new branch of cuprate superconductors.

The mechanism of superconductivity in cuprates remains one of the big challenges of condensed matter physics.High Tc cuprates crystallize into layered perovskite structure featuring copper oxygen octahedral coordination. Due to the Jahn Teller effect in combination with the strong static Coulomb interaction, the octahedra in high Tc cuprates are elongated along the c axis, leading to a 3dx2-y2 orbital at the top of the band structure wherein the doped holes reside.This scenario gives rise to two dimensional characteristics in high Tc cuprates that favor d wave pairing symmetry. Here we report superconductivity in a cuprate Ba2CuO4-y wherein the local octahedron is in a very exceptional compressed version.The Ba2CuO4-y compound was synthesized at high pressure at high temperatures, and shows bulk superconductivity with critical temperature Tc above 70 K at ambient conditions. This superconducting transition temperature is more than 30 K higher than the Tc for the isostructural counterparts based on classical La2CuO4. X-ray absorption measurements indicate the heavily doped nature of the Ba2CuO4-y superconductor. In compressed octahedron the 3d3z2-r2 orbital will be lifted above the 3dx2-y2 orbital, leading to significant three dimensional nature in addition to the conventional 3dx2-y2 orbital. This work sheds important new light on advancing our comprehensive understanding of the superconducting mechanism of high Tc in cuprate materials., Comment: Total 28 pages incuding 9 Figures

We present a detailed magnetic study of the triangular antiferromagnet ${\mathrm{ErMgGaO}}_{4}$. A point charge calculation under the single ion approximation reveals a crystal field ground state doublet with a strong Ising-like behavior of the ${\mathrm{Er}}^{3+}$ moment along the $c$ axis. Magnetic susceptibility and specific heat measurements indicate no presence of magnetic transitions above 0.1 K and no evidence of residual entropy as the temperature approaches zero. Zero field (ZF) $\ensuremath{\mu}\mathrm{SR}$ measurements shows no sign of static uniform or random field and longitudinal field (LF) $\ensuremath{\mu}\mathrm{SR}$ measurements exhibit persistent spin fluctuations down to our lowest temperature of 25 mK. Our results provide evidence of a quantum spin liquid state in the triangular antiferromagnet ${\mathrm{ErMgGaO}}_{4}$.

We present a muon spin relaxation study of the Mott transition in BaCoS_2 using two independent control parameters: (i) pressure p to tune the electronic bandwidth and (ii) Ni-substitution x on the Co site to tune the band filling. For both tuning parameters, the antiferromagnetic insulating state first transitions to an antiferromagnetic metal and finally to a paramagnetic metal without undergoing any structural phase transition. BaCoS_2 under pressure displays minimal change in the ordered magnetic moment S_ord until it collapses abruptly upon entering the antiferromagnetic metallic state at p_cr ~ 1.3 GPa. In contrast, S_ord in the Ni-doped system Ba(Co_{1-x}Ni_{x})S_{2} steadily decreases with increasing x until the antiferromagnetic metallic region is reached at x_cr ~ 0.22. In both cases, significant phase separation between magnetic and nonmagnetic regions develops when approaching p_cr or x_cr, and the antiferromagnetic metallic state is characterized by weak, random, static magnetism in a small volume fraction. No dynamical critical behavior is observed near the transition for either tuning parameter. These results demonstrate that the quantum evolution of both the bandwidth- and filling-controlled metal-insulator transition at zero temperature proceeds as a first-order transition. This behavior is common to magnetic Mott transitions in RENiO_3 and V_2O_3, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase., 8 pages, 6 Figures

Transition metal oxides of the $4d$ and $5d$ block have recently become the targets of materials discovery, largely due to their strong spin-orbit coupling that can generate exotic magnetic and electronic states. Here we report the high pressure synthesis of Lu$_2$Rh$_2$O$_7$, a new cubic pyrochlore oxide based on $4d^5$ Rh$^{4+}$ and characterizations via thermodynamic, electrical transport, and muon spin relaxation measurements. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetic contribution, while heat capacity shows an enhanced Sommerfeld coefficient, $\gamma$ = 21.8(1) mJ/mol-Rh K$^2$. Muon spin relaxation measurements confirm that Lu$_2$Rh$_2$O$_7$ remains paramagnetic down to 2 K. Taken in combination, these three measurements suggest that Lu$_2$Rh$_2$O$_7$ is a correlated paramagnetic metal with a Wilson ratio of $R_W = 2.5$. However, electric transport measurements present a striking contradiction as the resistivity of Lu$_2$Rh$_2$O$_7$ is observed to monotonically increase with decreasing temperature, indicative of a nonmetallic state. Furthermore, although the magnitude of the resistivity is that of a semiconductor, the temperature dependence does not obey any conventional form. Thus, we propose that Lu$_2$Rh$_2$O$_7$ may belong to the same novel class of non-Fermi liquids as the nonmetallic metal FeCrAs., Comment: 11 pages, 5 figures

We present muon spin rotation and relaxation (muSR) measurements as well as demagnetising field corrected magnetisation measurements on polycrystalline samples of the noncentrosymmetric superconductor BeAu. From muSR measurements in a transverse field, we determine that BeAu is a type-I superconductor with Hc = 256 Oe, amending the previous understanding of the compound as a type-II superconductor. To account for demagnetising effects in magnetisation measurements, we produce an ellipsoidal sample, for which a demagnetisation factor can be calculated. After correcting for demagnetising effects, our magnetisation results are in agreement with our muSR measurements. Using both types of measurements we construct a phase diagram from T = 30 mK to Tc = 3.25 K. We then study the effect of hydrostatic pressure and find that 450 MPa decreases Tc by 34 mK, comparable to the change seen in type-I elemental superconductors Sn, In and Ta, suggesting BeAu is far from a quantum critical point accessible by the application of pressure., 10 pages, 8 figures

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Muon spin rotation and relaxation studies have been performed on a "111" family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was characterized by obtaining the temperature and doping dependences of the local ordered magnetic moment size and the volume fraction of the magnetically ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long range magnetically ordered state is observed, while for higher x than 0.4 % magnetic order becomes more disordered and is completely suppressed for x = 1.5 %. The magnetic volume fraction continuously decreases with increasing x. The combination of magnetic and superconducting volumes implies that a spatially-overlapping coexistence of magnetism and superconductivity spans a large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction of both the ordered moment size and the volume fraction is observed below the superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron pnictides in which one of these two parameters exhibits a reduction below TC, but not both. The suppression of magnetic order is further enhanced with increased Ni doping, leading to a reentrant non-magnetic state below T_C for x = 1.3 %. The reentrant behavior indicates an interplay between antiferromagnetism and superconductivity involving competition for the same electrons. These observations are consistent with the sign-changing s-wave superconducting state, which is expected to appear on the verge of microscopic coexistence and phase separation with magnetism. We also present a universal linear relationship between the local ordered moment size and the antiferromagnetic ordering temperature TN across a variety of iron-based superconductors. We argue that this linear relationship is consistent with an itinerant-electron approach, in which Fermi surface nesting drives antiferromagnetic ordering.

The original version of this article omitted the following from the Acknowledgements: “CAM and AL were supported by the NSF MRSEC program through Columbia in the Center for Precision Assembly of Superstratic and Superatomic Solids (DMR-1420634). Additionally, this research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under ‘Contract No. DE-AC02-05CH11231’.” This has now been corrected in both the PDF and HTML versions of the article.

We report studies on the Ba2CuO4- y superconductor (1) with features including compressed local coordination and extreme over-doping level, etc.; while such behaviors are not favorable for superconductivity in previous cuprates (2), Yamamoto (3) raises an inquiry on one of these features: The compressed local octahedron. We have solid evidence to show that what we discussed in our paper has nothing to do with his speculation. There are 2 types of “214” structures with a CuO2 plane, i.e., the T phase based on La2CuO4 and the T′ phase based on Nd2CuO4 due to the different charge reservoir substructures, the rock-salt type for the former and the fluorite type for the latter. The relative strong Coulomb attraction interaction in the rock-salt layer over … [↵][1]1To whom correspondence may be addressed. Email: Jin{at}iphy.ac.cn. [1]: #xref-corresp-1-1

The van der Waals interfaces of molecular donor/acceptor or graphene-like two-dimensional (2D) semiconductors are central to concepts and emerging technologies of light-electricity interconversion. Examples include, among others, solar cells, photodetectors, and light emitting diodes. A salient feature in both types of van der Waals interfaces is the poorly screened Coulomb potential that can give rise to bound electron-hole pairs across the interface, i.e., charge transfer (CT) or interlayer excitons. Here we address common features of CT excitons at both types of interfaces. We emphasize the competition between localization and delocalization in ensuring efficient charge separation. At the molecular donor/acceptor interface, electronic delocalization in real space can dictate charge carrier separation. In contrast, at the 2D semiconductor heterojunction, delocalization in momentum space due to strong exciton binding may assist in parallel momentum conservation in CT exciton formation.

Recently a new diluted magnetic semiconductor, (Ba,K)(Zn,Mn)2As2 (BZA), with high Curie temperature was discovered, showing an independent spin and charge-doping mechanism. This makes BZA a promising material for spintronics devices. We report the successful growth of a BZA single crystal for the first time in this study. An Andreev reflection junction, which can be used to evaluate spin polarization, was fabricated based on the BZA single crystal. A 66% spin polarization of the BZA single crystal was obtained by Andreev reflection spectroscopy analysis.

Neutron diffraction and muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) studies are presented for the newly characterized polymorph of ${\mathrm{NiNb}}_{2}{\mathrm{O}}_{6} (\ensuremath{\beta}\ensuremath{-}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{6})$ with space group ${P4}_{2}/n$ and $\ensuremath{\mu}\mathrm{SR}$ data only for the previously known columbite structure polymorph with space group $Pbcn$. The magnetic structure of the ${P4}_{2}/n$ form was determined from neutron diffraction using both powder and single-crystal data. Powder neutron diffraction determined an ordering wave vector $\stackrel{P\vec}{k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$. Single-crystal data confirmed the same $\stackrel{P\vec}{k}$ vector and showed that the correct magnetic structure consists of antiferromagnetically coupled chains running along the $a$ or $b$ axis in adjacent ${\mathrm{Ni}}^{2+}$ layers perpendicular to the $c$ axis, which is consistent with the expected exchange interaction hierarchy in this system. The refined magnetic structure is compared with the known magnetic structures of the closely related trirutile phases, ${\mathrm{NiSb}}_{2}{\mathrm{O}}_{6}$ and ${\mathrm{NiTa}}_{2}{\mathrm{O}}_{6}$. $\ensuremath{\mu}\mathrm{SR}$ data finds a transition temperature of ${T}_{N}\ensuremath{\sim}15\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for this system, while the columbite polymorph exhibits a lower ${T}_{N}=5.7(3)$ K. Our $\ensuremath{\mu}\mathrm{SR}$ measurements also allowed us to estimate the critical exponent of the order parameter $\ensuremath{\beta}$ for each polymorph. We found $\ensuremath{\beta} =0.25(3)$ and 0.16(2) for the $\ensuremath{\beta}$ and columbite polymorphs, respectively. The single-crystal neutron scattering data give a value for the critical exponent $\ensuremath{\beta} =0.28(3)$ for $\ensuremath{\beta}\ensuremath{-}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{6}$, in agreement with the $\ensuremath{\mu}\mathrm{SR}$ value. While both systems have $\ensuremath{\beta}$ values less than 0.3, which is indicative of reduced dimensionality, this effect appears to be much stronger for the columbite system. In other words, although both systems appear to be well described by $S=1$ spin chains, the interchain interactions in the $\ensuremath{\beta}$ polymorph are likely much larger.

In its orthorhombic T d polymorph, MoTe2 is a type-II Weyl semimetal, where the Weyl fermions emerge at the boundary between electron and hole pockets. Non-saturating magnetoresistance and superconductivity were also observed in T d-MoTe2. Understanding the superconductivity in T d-MoTe2, which was proposed to be topologically non-trivial, is of eminent interest. Here, we report high-pressure muon-spin rotation experiments probing the temperature-dependent magnetic penetration depth in T d-MoTe2. A substantial increase of the superfluid density and a linear scaling with the superconducting critical temperature T c is observed under pressure. Moreover, the superconducting order parameter in T d-MoTe2 is determined to have 2-gap s-wave symmetry. We also exclude time-reversal symmetry breaking in the superconducting state with zero-field μSR experiments. Considering the strong suppression of T c in MoTe2 by disorder, we suggest that topologically non-trivial s +− state is more likely to be realized in MoTe2 than the topologically trivial s ++ state., Understanding the superconductivity in topological materials is of eminent interest. Here, Guguchia et al. report temperature-dependent magnetic penetration depth in the superconducting state of T d-MoTe2 under pressure, suggesting a topologically nontrivial s +− order parameter.

We present muon spin rotation and relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements on the noncentrosymmetric superconductor ${\mathrm{PbTaSe}}_{2}$. From measurements in an applied transverse field between ${H}_{c1}$ and ${H}_{c2}$, we extract the superfluid density as a function of temperature in the vortex state. These data can be fit with a fully gapped two-band model, consistent with previous evidence from ARPES, thermal conductivity, and resistivity. Furthermore, zero-field measurements show no evidence for a time-reversal symmetry-breaking field greater than 0.05 G in the superconducting state. This makes exotic fully gapped spin-triplet states unlikely, and hence we contend that ${\mathrm{PbTaSe}}_{2}$ is characterized by conventional BCS $s$-wave superconductivity in multiple bands.

Interfacial charge separation and recombination at heterojunctions of monolayer transition metal dichalcogenides (TMDCs) are of interest to two-dimensional optoelectronic technologies. These processes can involve large changes in parallel momentum vector due to the confinement of electrons and holes to the K valleys in each layer. Because these high-momentum valleys are usually not aligned across the interface of two TMDC monolayers, how parallel momentum is conserved in the charge separation or recombination process becomes a key question. Here we probe this question using the model system of a type-II heterojunction formed by MoS2 and WSe2 monolayers and the experimental technique of femtosecond pump–probe spectroscopy. Upon photoexcitation specifically of WSe2 at the heterojunction, we observe ultrafast (

We use neutron diffraction and muon spin relaxation to study the effect of in-plane uniaxial pressure on the antiferromagnetic (AF) orthorhombic phase in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ and its Co- and Ni-substituted members near optimal superconductivity. In the low-temperature AF ordered state, uniaxial pressure necessary to detwin the orthorhombic crystals also increases the magnetic ordered moment, reaching an 11% increase under 40 MPa for ${\mathrm{BaFe}}_{1.9}{\mathrm{Co}}_{0.1}{\mathrm{As}}_{2}$, and a 15% increase for ${\mathrm{BaFe}}_{1.915}{\mathrm{Ni}}_{0.085}{\mathrm{As}}_{2}$. We also observe an increase of the AF ordering temperature (${T}_{N}$) of about 0.25 K/MPa in all compounds, consistent with density functional theory calculations that reveal better Fermi surface nesting for itinerant electrons under uniaxial pressure. The doping dependence of the magnetic ordered moment is captured by combining dynamical mean field theory with density functional theory, suggesting that the pressure-induced moment increase near optimal superconductivity is closely related to quantum fluctuations and the nearby electronic nematic phase.

Spintronics holds the promise of novel functionality and higher device performance, but a massive challenge is to find semiconducting materials with good electronic properties that are also ferromagnetic. Dilute magnetic semiconductors (DMS) such as Mn-doped GaAs have great potential but are notoriously difficult to make, only stabilizing in thin films in most cases. Recently, a promising new class of DMS materials based on the chemistry and structure of iron-based superconductors has been discovered, including the compound (Ba,K)(Zn,Mn)${}_{2}$As${}_{2}$. These materials can be synthesized in bulk form, providing an unprecedented opportunity to study the mechanism of ferromagnetism in semiconductors. In this work, the authors present detailed temperature-dependent characterization of the atomic and magnetic structure of (Ba,K)(Zn,Mn)${}_{2}$As${}_{2}$ using x-ray and neutron pair distribution function techniques, establishing the existence of an unexpected structural distortion on a short length scale and surprisingly robust short-range ferromagnetic correlations that persist even at room temperature. These results fill in important gaps in the experimental understanding of DMS materials and provide valuable guidance to their theoretical description.