In iron-based superconductors the interactions driving the nematic order (that breaks four-fold rotational symmetry in the iron plane) may also mediate the Cooper pairing. The experimental ...determination of these interactions, which are believed to depend on the orbital or the spin degrees of freedom, is challenging because nematic order occurs at, or slightly above, the ordering temperature of a stripe magnetic phase. Here, we study FeSe (ref. )-which exhibits a nematic (orthorhombic) phase transition at Ts = 90 K without antiferromagnetic ordering-by neutron scattering, finding substantial stripe spin fluctuations coupled with the nematicity that are enhanced abruptly on cooling through Ts. A sharp spin resonance develops in the superconducting state, whose energy (∼4 meV) is consistent with an electron-boson coupling mode revealed by scanning tunnelling spectroscopy. The magnetic spectral weight in FeSe is found to be comparable to that of the iron arsenides. Our results support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations.
Magnetic ground state of FeSe Wang, Qisi; Shen, Yao; Pan, Bingying ...
Nature communications,
07/2016, Letnik:
7, Številka:
1
Journal Article
Recenzirano
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Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for establishing its pairing mechanism. The parent compounds of the cuprate and iron-pnictide superconductors ...exhibit Néel and stripe magnetic order, respectively. However, FeSe, the structurally simplest iron-based superconductor, shows nematic order (Ts=90 K), but not magnetic order in the parent phase, and its magnetic ground state is intensely debated. Here we report inelastic neutron-scattering experiments that reveal both stripe and Néel spin fluctuations over a wide energy range at 110 K. On entering the nematic phase, a substantial amount of spectral weight is transferred from the Néel to the stripe spin fluctuations. Moreover, the total fluctuating magnetic moment of FeSe is ∼60% larger than that in the iron pnictide BaFe2As2. Our results suggest that FeSe is a novel S=1 nematic quantum-disordered paramagnet interpolating between the Néel and stripe magnetic instabilities.
The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide (2H−TaS2) at low temperature is boosted by applying hydrostatic pressures to study both vibrational and ...magnetic transport properties. Around Pc, we observe a superconducting dome with a maximum superconducting transition temperature Tc=9.1 K. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods. While a d wave and single-gap BCS prediction cannot describe the lower critical field Hc1 data, the temperature dependence of the Hc1 can be well described by a single-gap anisotropic s-wave order parameter.
Magnetic Na titanate nanotubes (HRTEM)+ M−H loop of pure and doped ones.
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•Na titanate was prepared together with the Fe and Co doped in nanotubular form.•Pure Na titanate (3d0) ...revealed 1st time magnetic hysteresis loop at room temperature.•Fe doping increased the magnetization, while Co doping enhanced the coercivity.
The preparation of magnetic nanotubes opens new avenues in nanobiotechnology as a consequence of their multiple properties embedded within the same moiety. Here, we report on synthesis and characterization of titanate nanotubes and their Fe-/Co-doped by means of X-ray diffraction, high-resolution transmission electron microscopy, and magnetic studies. Although the absence of 3d elements in Na2Ti3O7 nanotubes, our data for the first time, exhibits room temperature M−H hysteresis. Furthermore, our results show that the magnetization increased with Fe doping, while Co doping enhanced the coercivity. This behaviour was expected from the magnetic character of Fe and the positive magnetocrystalline anisotropy of Co. These results give access to the doping effect on tuning the properties of Na2Ti3O7 nanotubes.
Zn
1-x
R
x
O (R = Li, Mg, Cr, Mn, Fe and Cd) were obtained by using co-precipitation synthesis technique with constant weight percent of 3% from R ions. The phase composition, crystal structure, ...morphology, density functional theory (DFT), and magnetic properties were examined to comprehend the influence of Zn
2+
partial substitution with R ions. X-ray diffraction shows that the ZnO lattice parameters were slightly affected by R doping and the doped sample crystallinity is enhanced. Our results show that introducing Cr, Mn and Fe along with Mg into ZnO induces a clear magnetic moment without any apparent distortion in the structural morphology. The spatial configuration of dopant atoms is determined from first-principles calculations, giving a better understanding of the position of the dopant atom responsible for the magnetism. The magnetic moments obtained from our calculations are 3.67, 5.0, and 4.33 μB per dopant atom for Cr, Mn, and Fe, respectively, which agree with the experimental values. While Cr and Fe tend to form clusters, Mn has more propensity to remain evenly distributed within the system, avoiding cluster-derived magnetism.
•Macroscopic LaNiO3 single crystals grown using optical floating-zone technique at high O2 pressure.•XRD and Laue diffraction indicate high quality of the single crystals.•Magnetisation and specific ...heat data indicate a lack of long range magnetic order in LaNiO3.
Macroscopic cm-sized single crystals of distorted perovskite LaNiO3 have been grown by means of the optical floating-zone method at oxygen pressures of 40 and 80 bar, respectively. Depending on the growth parameters, the crystals feature a transition to long-range antiferromagnetic order as indicated by magnetic susceptibility and specific heat studies or lack of magnetic order. Our findings resemble recent contradictory reports which either imply the presence of unexpected antiferromagnetism (Guo et al., 2018) or the absence of magnetic order (Zhang et al., 2017; Wang et al., 2018). Our data indicate that long-range magnetic order is not intrinsic to LaNiO3.
The modification and tailoring characteristics of nanostructured materials are of great interest due to controllable and unusual inherent properties in such materials. A simple spray pyrolysis ...technique was used to prepare pure and La-doped ZnO films. The influence of La concentration (0, 0.33, 0.45, 0.66, 0.92 and 1.04 at. %) on the structural, optical, and magnetic properties of ZnO was investigated. The exact nominal compositions of the prepared films were determined from the field emission scanning electron microscope occupied with EDX. X-ray diffraction confirmed that the samples possessed single-phase hexagonal wurtzite structure. The main crystal size was decreased from 315.50 Å to 229.04 Å depending on La dopant concentration. This decrease is due to the small ionic radius of Zn ions in compared to La ions. The band gap values were found to be depend strongly on La3+ ion content. Introducing La into ZnO induces a clear magnetic moment without any distortion in the geometrical symmetry, it also reveals the ferromagnetic coupling. The saturation magnetic moment of 1.04 at.% La-doped ZnO shows the highest value of 0.014 emu, which is ∼23 times higher than pure ZnO sample. The obtained results were discussed and compared with other literature data and showed an acceptable agreement.
•Pure and La doped ZnO thin films were synthesized in hexagonal phase.•Band gap values depended strongly on the La doping ratio.•La doping induced ferromagnetism in the paramagnetic ZnO.
We report the revised crystal structure, static and dynamic magnetic properties of quasi-two dimensional honeycomb-lattice silver delafossite Ag3Co2SbO6. The magnetic susceptibility and specific heat ...data are consistent with the onset of antiferromagnetic long range order at low temperatures with Néel temperature TN ∼ 21.2 K. In addition, the magnetization curves revealed a field-induced (spin-flop type) transition below TN in moderate magnetic fields. The GGA+U calculations show the importance of the orbital degrees of freedom, which maintain a hierarchy of exchange interaction in the system. The strongest antiferromagnetic exchange coupling was found in the shortest Co-Co pairs and is due to direct and superexchange interaction between the half-filled xz + yz orbitals pointing directly to each other. The other four out of six nearest neighbor exchanges within the cobalt hexagon are suppressed, since for these bonds the active half-filled orbitals turned out to be parallel and do not overlap. The electron spin resonance (ESR) spectra reveal a broad absorption line attributed to the Co(2+) ion in an octahedral coordination with an average effective g-factor g = 2.40 ± 0.05 at room temperature and show strong divergence of the ESR parameters below ∼150 K, which implies an extended region of short-range correlations. Based on the results of magnetic and thermodynamic studies in applied fields, we propose a magnetic phase diagram for the new honeycomb-lattice delafossite.
In low-dimensional electron systems, charge density waves (CDW) and superconductivity are two of the most fundamental collective quantum phenomena. For all known quasi-two-dimensional ...superconductors, the origin and exact boundary of the electronic orderings and superconductivity are still attractive problems. Through transport and thermodynamic measurements, we report on the field-temperature phase diagram in 2H-TaS2 single crystals. We show that the superconducting transition temperature (Tc) increases by one order of magnitude from temperatures at 0.98 K up to 9.15 K at 8.7 GPa when the Tc becomes very sharp. Additionally, the effects of 8.7 GPa illustrate a suppression of the CDW ground state, with critically small Fermi surfaces. Below the Tc the lattice of magnetic flux lines melts from a solid-like state to a broad vortex liquid phase region. Our measurements indicate an unconventional s-wave-like picture with two energy gaps evidencing its multi-band nature.
We investigate magnetoelectric coupling and low-energy magnetic excitations in multiferroic α-Cu2V2O7 by detailed thermal expansion, magnetostriction, specific heat and magnetization measurements in ...magnetic fields up to 15 T and by high-field/high-frequency electron spin resonance studies. Our data show negative thermal expansion in the temperature range ≤200 K under study. Well-developed anomalies associated with the onset of multiferroic order (canted antiferromagnetism with a significant magnetic moment and ferroelectricity) imply pronounced coupling to the structure. We detect anomalous entropy changes in the temperature regime up to ∼80 K which significantly exceed the spin entropy. Failure of Grüneisen scaling further confirms that several dominant ordering phenomena are concomitantly driving the multiferroic order. By applying external magnetic fields, anomalies in the thermal expansion and in the magnetization are separated. Noteworthy, the data clearly imply the development of a canted magnetic moment at temperatures above the structural anomaly. Low-field magnetostriction supports the scenario of exchange-striction driven multiferroicity. We observe low-energy magnetic excitations well below the antiferromagnetic gap, i.e., a ferromagnetic-type resonance branch associated with the canted magnetic moment arising from Dzyaloshinsii–Moriya (DM) interactions. The anisotropy parameter \(\tilde{D}=1.6(1)\) meV indicates a sizeable ratio of DM- and isotropic magnetic exchange.
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