Neutron crystal-field spectroscopy in underdoped and overdoped copper oxide superconductors

In many cuprate superconductors rare-earth (R) ions can be placed close to the superconducting copper oxide planes; thus, the crystal-field interaction at the R site constitutes an ideal probe of the local symmetry as well as the local charge distribution and thereby monitors directly changes of the carrier concentration induced by doping. For several compounds the crystal-field spectra observed by inelastic neutron scattering separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for the formation of clusters which make the systems inhomogeneous. Moreover, it is found that the intrinsic linewidths of crystal-field transitions vary with temperature, which is essentially a reflection of the density of states associated with the charge carriers at the Fermi energy. Linewidth studies can therefore reveal information about the energy gap. For underdoped systems there is evidence for the formation of a pseudogap atT* >T

Evidence for Large Electric Polarization From Collinear Magnetism in TmMnO\u3csub\u3e3\u3c/sub\u3e

There has been tremendous research activity in the field of magneto-electric (ME) multiferroics after Kimura et al (2003 Nature 426 55) showed that antiferromagnetic and ferroelectric orders coexist in orthorhombically distorted perovskite TbMnO3 and are strongly coupled. It is now generally accepted that ferroelectricity in TbMnO3 is induced by magnetic long-range order that breaks the symmetry of the crystal and creates a polar axis (Kenzelmann et al 2005 Phys. Rev. Lett. 95 087206). One remaining key question is whether magnetic order can induce ferroelectric polarization that is as large as that of technologically useful materials. We show that ferroelectricity in orthorhombic (o) TmMnO3 is induced by collinear magnetic order, and that the lower limit for its electric polarization is larger than in previously investigated orthorhombic heavy rare-earth manganites. The temperature dependence of the lattice constants provides further evidence of large spin–lattice coupling effects. Our experiments suggest that the ferroelectric polarization in the orthorhombic perovskites with commensurate magnetic ground states could pass the 1 μC cm-2 threshold, as predicted by theory (Sergienko et al 2006 Phys. Rev. Lett. 97 227204; Picozzi et al 2007 Phys. Rev. Lett. 99 227201)

Field-induced hexagonal to square transition of the vortex lattice in overdoped La{1.8}Sr{0.2}CuO{4}

We report on a small angle neutron scattering study of the vortex lattice in overdoped La{2-x}Sr{x}CuO{4} (x=0.2) up to high magnetic fields (9.5 Tesla) applied perpendicular to the CuO2 planes. At low magnetic fields we observe a crossover from hexagonal to square coordination of the vortex lattice. This field-induced transition confirms the results obtained in slightly overdoped La{2-x}Sr{x}CuO{4} (x=0.17).Comment: 3 pages, 2 figures. to appear in Physica C (proceedings for the M2S-HTSC-VII Conference, May 25-30, Rio de Janeiro

Tuning the metal-insulator transition in NdNiO3{\mathrm{NdNiO}}_{3} heterostructures via Fermi surface instability and spin fluctuations

We employed in situ pulsed laser deposition (PLD) and angle-resolved photoemission spectroscopy (ARPES) to investigate the mechanism of the metal-insulator transition (MIT) in NdNiO3 (NNO) thin films, grown on NdGaO3(110) and LaAlO3(100) substrates. In the metallic phase, we observe three-dimensional hole and electron Fermi surface (FS) pockets formed from strongly renormalized bands with well-defined quasiparticles. Upon cooling across the MIT in NNO/NGO sample, the quasiparticles lose coherence via a spectral weight transfer from near the Fermi level to localized states forming at higher binding energies. In the case of NNO/LAO, the bands are apparently shifted upward with an additional holelike pocket forming at the corner of the Brillouin zone. We find that the renormalization effects are strongly anisotropic and are stronger in NNO/NGO than NNO/LAO. Our study reveals that substrate-induced strain tunes the crystal field splitting, which changes the FS properties, nesting conditions, and spin-fluctuation strength, and thereby controls the MIT via the formation of an electronic order parameter with QAF∼(1/4,1/4,1/4±δ)

Rotation symmetry breaking in La2−xSrxCuO4{\mathrm{La}}_{2-x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4} revealed by angle-resolved photoemission spectroscopy

Using angle-resolved photoemission spectroscopy it is revealed that in the vicinity of optimal doping the electronic structure of La2−xSrxCuO4 cuprate undergoes an electronic reconstruction associated with a wave vector qa=(π,0). The reconstructed Fermi surface and folded band are distinct to the shadow bands observed in BSCCO cuprates and in underdoped La2−xSrxCuO4 with x≤0.12, which shift the primary band along the zone diagonal direction. Furthermore, the folded bands appear only with qa=(π,0) vector, but not with qb=(0,π). We demonstrate that the absence of qb reconstruction is not due to the matrix-element effects in the photoemission process, which indicates the fourfold symmetry is broken in the system

Selective probing of hidden spin-polarized states in inversion-symmetric bulk MoS2{\mathrm{MoS}}_{2}

Spin- and angle-resolved photoemission spectroscopy is used to reveal that a large spin polarization is observable in the bulk centrosymmetric transition metal dichalcogenide MoS2. It is found that the measured spin polarization can be reversed by changing the handedness of incident circularly polarized light. Calculations based on a three-step model of photoemission show that the valley and layer-locked spin- polarized electronic states can be selectively addressed by circularly polarized light, therefore providing a novel route to probe these hidden spin-polarized states in inversion-symmetric systems as predicted by Zhang et al. [Nat. Phys. 10, 387 (2014).]

Proximity-Induced Novel Ferromagnetism Accompanied with Resolute Metallicity in NdNiO3 Heterostructure

Employing X-ray magnetic circular dichroism (XMCD), angle-resolved photoemission spectroscopy (ARPES), and momentum-resolved density fluctuation (MRDF) theory, the magnetic and electronic properties of ultrathin NdNiO3 (NNO) film in proximity to ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) layer are investigated. The experimental data shows the direct magnetic coupling between the nickelate film and the manganite layer which causes an unusual ferromagnetic (FM) phase in NNO. Moreover, it is shown the metal–insulator transition in the NNO layer, identified by an abrupt suppression of ARPES spectral weight near the Fermi level (EF), is absent. This observation suggests that the insulating AFM ground state is quenched in proximity to the FM layer. Combining the experimental data (XMCD and AREPS) with the momentum-resolved density fluctuation calculation (MRDF) reveals a direct link between the MIT and the magnetic orders in NNO systems. This work demonstrates that the proximity layer order can be broadly used to modify physical properties and enrich the phase diagram of RENiO3 (RE = rare-earth element)