Antiferromagnetic Parent Compounds Research Articles

Structural phase transition and potential superconductivity initiated by pressure-driven in 1T–CrSe2

The exploration of the superconducting properties of antiferromagnetic parent compounds containing transition metals under pressure provides a unique idea for finding and designing superconducting materials with better performance. In this paper, the close relationship between the possible superconductivity and structure phase transition of the typical van der Waals layered material 1 T–CrSe2 induced by pressure is studied by means of electrical transport and x-ray diffraction for the first time. We introduce the possibility of pressure-induced superconductivity at 20 GPa, with a critical T c of approximately at 4 K. The superconductivity persists up to the highest measured pressure of 70 GPa, with a maximum T c ∼ 5 K at 24 GPa. We observed a structure phase transition from P-3 m1 to C2/m space group in the range of 9.4–11.7 GPa. The results show that the structural phase transition leads to the metallization of 1 T–CrSe2 and the further pressure effect makes the superconductivity appear in the new structure. The material undergoes a transition from a two-dimensional layered structure to a three-dimensional structure under pressure. This is the first time that possible superconductivity has been observed in 1 T–CrSe2.

Synthesis and Magnetic, Optical, and Electrocatalytic Properties of High-Entropy Mixed-Metal Tungsten and Molybdenum Oxides

High-entropy oxides (HEOs) are of interest for their unique physical and chemical properties. Significant lattice distortions, strain, and tolerance for high-vacancy concentrations set HEOs apart from single-metal or mixed-metal oxides. Herein, we synthesized and characterized the structures and compositions, along with the optical, magnetic, and electrocatalytic properties, of two families of high-entropy mixed-metal tungsten and molybdenum oxides, AWO4 and B2Mo3O8, where A and B are 3d transition metals. The HEOs A6WO4 (A = Mn, Fe, Co, Ni, Cu, and Zn) and B25Mo3O8 (B = Mn, Fe, Co, Ni, and Zn), as well as all accessible single-metal AWO4 and B2Mo3O8 parent compounds, were synthesized using high-temperature solid-state methods. X-ray photoelectron spectroscopy analysis of the surfaces revealed that the HEOs largely had the metal oxidation states expected from the bulk chemical formulas, but in some cases they were different than in the parent compounds. A6WO4 exhibited antiferromagnetic (AFM) ordering with a Néel temperature of 30 K, which is less than the average of its AFM parent compounds, and had a narrow band gap of 0.24 eV, which is much lower than all of its parent compounds. B25Mo3O8 was paramagnetic, despite the existence of AFM and ferromagnetic ordering in several of its parent compounds and had no observable band gap, which is analogous to its parent compounds. Both A6WO4 and B25Mo3O8 exhibited superior catalytic activity relative to the parent compounds for the oxygen evolution reaction, the oxidation half reaction of overall water splitting, under alkaline conditions, based on the overpotential required to reach the benchmark surface area normalized current density. Consistent with literature predictions of OER durability for ternary tungsten and molybdenum oxides, A6WO4 and B25Mo3O8 also exhibited stable performance without significant dissolution during 10 h stability experiments at a constant current.

Determining the electron-phonon coupling in superconducting cuprates by resonant inelastic x-ray scattering: Methods and results on Nd1+xBa2−xCu3O7−δ

The coupling between lattice vibration quanta and valence electrons can induce charge density modulations and decisively influence the transport properties of materials, e.g. leading to conventional superconductivity. In high critical temperature superconductors, where electronic correlation is the main actor, the actual role of electron-phonon coupling (EPC) is being intensely debated theoretically and investigated experimentally. We present an in-depth study of how the EPC strength can be obtained directly from resonant inelastic x-ray scattering (RIXS) data through the theoretical approach derived by Ament et al. [EPL 95, 27008 (2011)]. The role of the model parameters (e.g. phonon energy $\omega_0$, intermediate state lifetime $1/\Gamma$, EPC matrix element $M$, and detuning energy $\Omega$) is thoroughly analyzed, providing general relations among them that can be used to make quantitative estimates of the dimensionless EPC $g = (M/\omega_0)^2$ without detailed microscopic modeling. We then apply these methods to very high resolution Cu $L_3$ edge RIXS spectra of three Nd$_{1+x}$Ba$_{2-x}$Cu$_3$O$_{7-\delta}$ films. For the insulating antiferromagnetic parent compound the value of $M$ as a function of the in-plane momentum transfer is obtained for Cu-O bond-stretching (breathing) and bond-bending (buckling) phonon branches. For the underdoped and the nearly optimally doped samples, the effects of Coulomb screening and of charge-density-wave correlations on $M$ are assessed. In light of the anticipated further improvements of the RIXS experimental resolution, this work provides a solid framework for an exhaustive investigation of the EPC in cuprates and other quantum materials.

Robustness of magnons near the quantum critical point in the heavy-fermion superconductor CeCu2Si2

Paramagnons are supposed to provide the pairing glue for unconventional superconductors. For the heavy-fermion superconductor ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$, there is indeed good evidence from inelastic neutron scattering (INS) that spin fluctuations drive the superconductivity. Here, we present the INS measurement of the inelastic response of the antiferromagnetic parent compound, `A-type' ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$, to probe the relation to the excitations of the superconducting (`S-type') sample. We find that the dispersion is very similar in the antiferromagnetic state and in the normal state of the superconducting sample. Pronounced differences to the response in the superconducting state exist at low energies around the zone center. These findings are in line with observations of other unconventional superconductors.

Influence of apical oxygen on the extent of in-plane exchange interaction in cuprate superconductors

In high Tc superconductors the magnetic and electronic properties are determined by the probability that valence electrons virtually jump from site to site in the CuO2 planes, a mechanism opposed by on-site Coulomb repulsion and favored by hopping integrals. The spatial extent of the latter is related to transport properties, including superconductivity, and to the dispersion relation of spin excitations (magnons). Here, for three antiferromagnetic parent compounds (single-layer Bi2Sr0.99La1.1CuO6+delta, double-layer Nd1.2Ba1.8Cu3O6 and infinite-layer CaCuO2) differing by the number of apical atoms, we compare the magnetic spectra measured by resonant inelastic x-ray scattering over a significant portion of the reciprocal space and with unprecedented accuracy. We observe that the absence of apical oxygens increases the in-plane hopping range and, in CaCuO2, it leads to a genuine 3D exchange-bond network. These results establish a corresponding relation between the exchange interactions and the crystal structure, and provide fresh insight into the materials dependence of the superconducting transition temperature.

Enhancement of magnetocaloric effect in V doped intermetallic compounds [formula omitted] (x=0, 0.20 and 0.35)

We report a large enhancement of magnetocaloric entropy change in vanadium substituted intermetallic compounds Nd(Co1−xVx)2Si2 (x = 0.20 and 0.35). The antiferromagnetic parent compound NdCo2Si2 exhibits inverse magnetocaloric response. Upon vanadium doping, antiferromagnetic transition is gradually suppressed and there is a drastic modification of the ground state, with appearance of ferromagnetism in Nd(Co0.65V0.35)2Si2. Such modifications are reflected in the magnetocaloric properties of the parent and the doped compounds. In comparison to the inverse magnetocaloric response of the parent compound, the doped compound with x = 0.35 shows a conventional magnetocaloric effect that is large in magnitude compared to the parent compound. A table-like MCE in a wide-temperature range of 5–60K have been observed in Nd(Co0.65V0.35)2Si2. The present study demonstrates enhancement of magnetic entropy by way of substitution at the transition metal site in these compounds.

Effect of Co and Ni substitution on the two magnetostructural phase transitions inFe1.12Te

Here we present the results of high-resolution x-ray diffraction experiments along with specific heat, resistivity, and magnetization measurements of chemically well-characterized ${\mathrm{Fe}}_{1.12\ensuremath{-}x}{M}_{x}\mathrm{Te}$ ($M=\mathrm{Co}$, Ni) samples. The motivation is to investigate how the two coupled magnetostructural phase transitions in the antiferromagnetic parent compound ${\mathrm{Fe}}_{1.12}\mathrm{Te}$ of chalcogenide superconductors can be tuned. While the two-step magnetostructural transition (tetragonal-to-orthorhombic followed by orthorhombic-to-monoclinic) persists in ${\mathrm{Fe}}_{1.10}{\mathrm{Co}}_{0.02}\mathrm{Te}$, only one, tetragonal-to-orthorhombic transition was observed in ${\mathrm{Fe}}_{1.10}{\mathrm{Ni}}_{0.02}\mathrm{Te}$. Upon increasing the Co and Ni substitution, the structural phase transitions and the long-range magnetic order are systematically suppressed without any sign of superconductivity. For high substitution levels $(x\ensuremath{\ge}0.05)$, a spin-glass-like behavior was observed and the low-temperature structure remains tetragonal. From our results, it can be inferred that the electron doping strongly suppresses the magnetostructural phase transitions.

Short-range cluster spin glass near optimal superconductivity inBaFe2−xNixAs2

High-temperature superconductivity in iron pnictides occurs when electrons are doped into their antiferromagnetic (AF) parent compounds. In addition to inducing superconductivity, electron-doping also changes the static commensurate AF order in the undoped parent compounds into short-range incommensurate AF order near optimal superconductivity. Here we use neutron scattering to demonstrate that the incommensurate AF order in BaFe$_{2-x}$Ni$_{x}$As$_{2}$ is not a spin-density-wave arising from the itinerant electrons in nested Fermi surfaces, but consistent with a cluster spin glass in the matrix of the superconducting phase. Therefore, optimal superconductivity in iron pnictides coexists and competes with a mesoscopically separated cluster spin glass phase, much different from the homogeneous coexisting AF and superconducting phases in the underdoped regime.

Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides.

High-temperature superconductivity in iron pnictides occurs when electrons and holes are doped into their antiferromagnetic parent compounds. Since spin excitations may be responsible for electron pairing and superconductivity, it is important to determine their electron/hole-doping evolution and connection with superconductivity. Here we use inelastic neutron scattering to show that while electron doping to the antiferromagnetic BaFe2As2 parent compound modifies the low-energy spin excitations and their correlation with superconductivity (<50 meV) without affecting the high-energy spin excitations (>100 meV), hole-doping suppresses the high-energy spin excitations and shifts the magnetic spectral weight to low-energies. In addition, our absolute spin susceptibility measurements for the optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above Tc can account for the superconducting condensation energy. These results suggest that high-Tc superconductivity in iron pnictides is associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons.

Competition between commensurate and incommensurate magnetic ordering in Fe1+yTe

The Fe${}_{1+y}$Te${}_{1\ensuremath{-}x}$Se${}_{x}$ compounds belong to the family of iron-based high-temperature superconductors, in which superconductivity often appears upon doping antiferromagnetic parent compounds. Unlike other Fe-based superconductors (in which the antiferromagnetic order is at the Fermi-surface nesting wave vector $[\frac{1}{2},\frac{1}{2},1]$), Fe${}_{1+y}$Te orders at a different wave vector, $[\frac{1}{2},0,\frac{1}{2}]$. Furthermore, the ordering wave vector depends on $y$, the occupation of interstitial sites with excess iron; the origin of this behavior is controversial. Using inelastic neutron scattering on Fe${}_{1.08}$Te, we find incommensurate magnetic fluctuations above the N\'eel temperature, even though the ordered state is bicollinear and commensurate with gapped spin waves. This behavior can be understood in terms of a competition between commensurate and incommensurate order, which we explain as a lock-in transition caused by the magnetic anisotropy.

Quasiparticle interference in antiferromagnetic parent compounds of iron-based superconductors

Recently reported quasiparticle interference imaging in underdoped Ca(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ shows pronounced $C$${}_{2}$ asymmetry, which is interpreted as an indication of an electronic nematic phase with a unidirectional electron band, dispersive predominantly along the $b$ axis of this orthorhombic material. On the other hand, even more recent transport measurements on untwinned samples show near isotropy of the resistivity in the $\mathit{ab}$ plane, with slightly larger conductivity along $a$ (and not $b$). We show that, in fact, both sets of data are consistent with the calculated ab initio Fermi surfaces, which have a decisively broken $C$${}_{4}$ and yet similar Fermi velocity in both directions. This reconciles completely the apparent contradiction between the conclusions of the scanning tunneling microscopy and the transport experiments.

Magnetic order close to superconductivity in the iron-based layered LaO1-xF x FeAs systems

Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

EFG calculations for Cu 2 + compounds

The electric field gradients at Cu in various binary and ternary Cu2+ oxides and halides, including several antiferromagnetic parent compounds of high-Tc superconductors, have been calculated with the full-potential linearized augmented plane wave method. The data from NOR are very well reproduced with the inclusion of Coulomb-correlation corrections (U). Not even an approximate agreement is obtained without U. Also many other properties of the 14 solids investigated are well accounted for.

Resonance in the electron-doped high-transition-temperature superconductor Pr0.88LaCe0.12CuO4-δ

In conventional superconductors, the interaction that pairs the electrons to form the superconducting state is mediated by lattice vibrations (phonons). In high-transition-temperature (high-T(c)) copper oxides, it is generally believed that magnetic excitations might play a fundamental role in the superconducting mechanism because superconductivity occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Indeed, a sharp magnetic excitation termed 'resonance' has been observed by neutron scattering in a number of hole-doped materials. The resonance is intimately related to superconductivity, and its interaction with charged quasi-particles observed by photoemission, optical conductivity, and tunnelling suggests that it might play a part similar to that of phonons in conventional superconductors. The relevance of the resonance to high-T(c) superconductivity, however, has been in doubt because so far it has been found only in hole-doped materials. Here we report the discovery of the resonance in electron-doped superconducting Pr0.88LaCe0.12CuO4-delta (T(c) = 24 K). We find that the resonance energy (E(r)) is proportional to T(c) via E(r) approximately 5.8k(B)T(c) for all high-T(c) superconductors irrespective of electron- or hole-doping. Our results demonstrate that the resonance is a fundamental property of the superconducting copper oxides and therefore must be essential in the mechanism of superconductivity.

Coexistence of magnetism and superconductivity in ultraclean underdopedYBa2Cu3O6.37

Muon spin rotation and magnetization measurements in ultraclean single crystals of superconducting ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ $(x=0.375)$ reveal transitions at $T=19$ K to a bulk superconducting state and below $T=15$ K to a disordered antiferromagnetic state. The superconducting state pins magnetic flux. At the lowest temperatures an internal magnetic field with a magnitude close to that seen in the antiferromagnetic parent compound is present throughout the bulk of the sample. The two states coexist on a nanometer length scale, and over a narrow region of oxygen doping.

Temperature dependence of the sublattice magnetization of the infinite-layer antiferromagnetSrCuO2

The ${}^{63}\mathrm{Cu},$ ${}^{65}\mathrm{Cu}$ nuclear-magnetic-resonance spectra and magnetic susceptibility were measured for the infinite-layer antiferromagnetic copper oxide ${\mathrm{SrCuO}}_{2}.$ The N\'eel temperature and effective magnetic moment were determined to be 442(6) K and $1.13(7){\ensuremath{\mu}}_{B},$ respectively. The antiferromagnetic structure has a quasi-two-dimensional character similar to other antiferromagnetic parent compounds of high-${T}_{C}$ superconductors.

DFT‐based solution to the gap problem of antiferromagnetic transition metal oxides and parent compounds of high‐Tc superconductors

AbstractCurrently used approximations to the exchange‐correlation potential in density functional theory (DFT) are known to fail in describing the properties of certain compounds, of which we discuss here only two examples: CoO and stoichiometric La2CuO4. Both materials are insulating and antiferromagnetic. A DFT calculation on CoO yields antiferromagnetic order, but Co‐associated magnetic moments that are by ∼1 μB smaller than the experimental value, and one obtains the electronic structure of a metal. The latter applies also to La2CuO4, and, in contrast to the experiment, the calculation does not even yield nonzero moments associated with the Cu atoms. We exploit the fact that approximate exchange‐correlation potentials lead necessarily to spin‐dependent densities that differ from the exact ones. We therefore derive modified Kohn–Sham (KS) equations in which the effective potentials depend on the exact spin densities rather than on the standard KS densities. If the latter are modified by adding small (spin‐up, spin‐down) portions that individually integrate to zero within the lattice unit cell and do not change the total charge density, the inconsistencies with the experiment can be removed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Resonance as a measure of pairing correlations in the high-Tc superconductor YBa2Cu3O6.6

One of the most striking properties of the high-transition-temperature (high-Tc) superconductors is that they are all derived from insulating antiferromagnetic parent compounds. The intimate relationship between magnetism and superconductivity in these copper oxide materials has intrigued researchers from the outset, because it does not exist in conventional superconductors. Evidence for this link comes from neutron-scattering experiments that show the unambiguous presence of short-range antiferromagnetic correlations (excitations) in the high-Tc superconductors. Even so, the role of such excitations in the pairing mechanism for superconductivity is still a subject of controversy. For YBa2Cu3O(6+x), where x controls the hole-doping level, the most prominent feature in the magnetic excitation spectrum is a sharp resonance (refs 6-11). Here we show that for underdoped YBa2Cu3O6.6, where x and Tc are below their optimal values, modest magnetic fields suppress the resonance significantly, much more so for fields approximately perpendicular to the CuO2 planes than for parallel fields. Our results indicate that the resonance measures pairing and phase coherence, suggesting that magnetism plays an important role in high-Tc superconductivity. The persistence of a field effect above Tc favours mechanisms in which the superconducting electron pairs are pre-formed in the normal state of underdoped copper oxide superconductors, awaiting transition to the superconducting state.

Finite Ferromagnetic Clusters La1−xZnxMnO3 (x = 0.05 and 0.10)

Detailed magnetic and nonlinear susceptibility measurements of the La1−xZn x MnO3 system are reported. For x = 0.05 and 0.1, a typical ferromagnetic (FM) signal in the field-cooled curve is observed at 38 K. However, for the insulating antiferromagnetic (AFM) parent compound LaMnO3 and for the x = 0.2 and 0.33 samples, the magnetization curves in the low temperature range, are smooth and flat. This observation confirms the coexistence of finite FM clusters in the insulating AFM region below the critical concentration (x c = 0.16) for percolation, predicted by the percolation model.

High pressure μSR studies: rare earths and related materials

After a short introduction to μSR with respect to the study of magnetic properties, followed by a brief outline of the principle of the high pressure-low temperature μSR spectrometer installed at the Paul Scherrer Institute, we discuss some measurements on rare earth materials employing this instrument. They are concerned with: (1) The pressure dependence of the spin turning process in ferromagnetic Gd. (2) The volume dependence of the internal magnetic field in the heavy rare earth metals Gd, Dy, and Ho in their ordered magnetic states. (3) The response of the (first order) magnetic transition in the frustrated antiferromagnets of type RMn2 (R = Y,Gd) to pressure. (4) The variation of magnetic parameters with pressure in La2CuO4 (powder sample), the antiferromagnetic parent compound of the high TC superconductors of type La2−x(Sr, Ba)xCuO4. In conclusion a short outlook on further developments is given.