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Experimental observation of incoherent-coherent crossover and orbital-dependent band renormalization in iron chalcogenide superconductors
The level of electronic correlation has been one of the key questions in understanding the nature of superconductivity. Among the iron-based superconductors, the iron chalcogenide family exhibits the strongest electron correlations. To gauge the correlation strength, we performed a systematic angle-resolved photoemission spectroscopy study on the iron chalcogenide series Fe 1+y Se x Te 1-x (0 < x < 0.59), a model system with the simplest structure. Our measurement reveals an incoherent-to-coherent crossover in the electronic structure as the selenium ratio increases and the system evolves from a weakly localized to a more itinerant state. Furthermore, we found that the effective mass of bands dominated by the d xy orbital character significantly decreases with increasing selenium ratio, as compared to the d xz /d yz orbital-dominated bands. The orbital-dependent change in the correlation level agrees with theoretical calculations on the band structure renormalization, and may help to understand the onset of superconductivity in Fe 1+y Se x Te 1-x .
Quantum oscillations in a bilayer with broken mirror symmetry: A minimal model for YBa 2 Cu 3 O 6 + δ
Using an exact numerical solution and semiclassical analysis, we investigate quantum oscillations (QOs) in a model of a bilayer system with an anisotropic (elliptical) electron pocket in each plane. Key features of QO experiments in the high temperature superconducting cuprate YBCO can be reproduced by such a model, in particular the pattern of oscillation frequencies (which reflect “magnetic breakdown” between the two pockets) and the polar and azimuthal angular dependence of the oscillation amplitudes. However, the requisite magnetic breakdown is possible only under the assumption that the horizontal mirror plane symmetry is spontaneously broken and that the bilayer tunneling t ⊥ is substantially renormalized from its ‘bare’ value. Lastly, under the assumption that t ⊥ = $\sim\atop{Z}_t$ $(0)\atop{⊥}$, where $\sim\atop{Z}$ is a measure of the quasiparticle weight, this suggests that $\sim\atop{Z}$ ≲ 1/20. Detailed comparisons with new YBa 2 Cu 3 O 6.58 QO data, taken over a very broad range of magnetic field, confirm specific predictions made by the breakdown scenario.
Variation in superconducting transition temperature due to tetragonal domains in two-dimensionally doped SrTiO 3
Strontium titanate is a low-temperature, non-Bardeen-Cooper-Schrieffer superconductor that superconducts to carrier concentrations lower than in any other system and exhibits avoided ferroelectricity at low temperatures. Neither the mechanism of superconductivity in strontium titanate nor the importance of the structure and dielectric properties for the superconductivity are well understood. We studied the effects of twin structure on superconductivity in a 5.5-nm-thick layer of niobium-doped SrTiO3 embedded in undoped SrTiO3. We used a scanning superconducting quantum interference device susceptometer to image the local diamagnetic response of the sample as a function of temperature. We observed regions that exhibited a superconducting transition temperature T-c greater than or similar to 10% higher than the temperature at which the sample was fully superconducting. The pattern of these regions varied spatially in a manner characteristic of structural twin domains. Some regions are too wide to originate on twin boundaries; therefore, we propose that the orientation of the tetragonal unit cell with respect to the doped plane affects T-c. Our results suggest that the anisotropic dielectric properties of SrTiO3 are important for its superconductivity and need to be considered in any theory of the mechanism of the superconductivity.
Nonequilibrium lattice-driven dynamics of stripes in nickelates using time-resolved x-ray scattering
Here, we investigate the lattice coupling to the spin and charge orders in the striped nickelate, La 1.75 Sr 0.25 NiO 4 , using time-resolved resonant x-ray scattering. Lattice-driven dynamics of both spin and charge orders are observed when the pump photon energy is tuned to that of an E u bond- stretching phonon. We present a likely scenario for the behavior of the spin and charge order parameters and its implications using a Ginzburg-Landau theory.
Observations of Continuum Depression in Warm Dense Matter with X-Ray Thomson Scattering
Detailed measurements of the electron densities, temperatures, and ionization states of compressed CH shells approaching pressures of 50 Mbar are achieved with spectrally resolved x-ray scattering. Laser- produced 9 keV x-rays probe the plasma during the transient state of three-shock coalescence. High signal- to-noise x-ray scattering spectra show direct evidence of continuum depression in highly degenerate warm dense matter states with electron densities $n_e$ > 10 24 cm –3 . Furthermore, the measured densities and temperatures agree well with radiation-hydrodynamic modeling when accounting for continuum lowering in calculations that employ detailed configuration accounting.
Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and Ab Initio Simulations
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Precise Quantization of the Anomalous Hall Effect near Zero Magnetic Field
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Inequivalence of Single-Particle and Population Lifetimes in a Cuprate Superconductor
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Electrically Tunable Magnetism in Magnetic Topological Insulators
The external controllability of the magnetic properties in topological insulators would be important both for fundamental and practical interests. Here we predict the electric-field control of ferromagnetism in a thin film of insulating magnetic topological insulators. The decrease of band inversion by the application of electric fields results in a reduction of magnetic susceptibility, and hence in the modification of magnetism. Remarkably, the electric field could even induce the magnetic quantum phase transition from ferromagnetism to paramagnetism. We further propose a transistor device in which the dissipationless charge transport of chiral edge states is controlled by an electric field. In particular, the field-controlled ferromagnetism in a magnetic topological insulator can be used for voltage based writing of magnetic random access memories in magnetic tunnel junctions. The simultaneous electrical control of magnetic order and chiral edge transport in such devices may lead to electronic and spintronic applications for topological insulators.
X-ray Detection of Transient Magnetic Moments Induced by a Spin Current in Cu
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Observation of the Zero Hall Plateau in a Quantum Anomalous Hall Insulator
We report experimental investigations on the quantum phase transition between the two opposite Hall plateaus of a quantum anomalous Hall insulator. We observe a well-defined plateau with zero Hall conductivity over a range of magnetic field around coercivity when the magnetization reverses. The features of the zero Hall plateau are shown to be closely related to that of the quantum anomalous Hall effect, but its temperature evolution exhibits a significant difference from the network model for a conventional quantum Hall plateau transition. We propose that the chiral edge states residing at the magnetic domain boundaries, which are unique to a quantum anomalous Hall insulator, are responsible for the novel features of the zero Hall plateau.
Intrinsic quantum anomalous Hall effect in the kagome lattice Cs 2 LiMn 3 F 12
In a kagome lattice, the time reversal symmetry can be broken by a staggered magnetic flux emerging from ferromagnetic ordering and intrinsic spin-orbit coupling, leading to several well-separated nontrivial Chern bands and intrinsic quantum anomalous Hall effect. Based on this idea and ab initio calculations, we propose the realization of the intrinsic quantum anomalous Hall effect in the single layer Cs 2 Mn 3 F 12 kagome lattice and on the (001) surface of a Cs 2 LiMn 3 F 12 single crystal by modifying the carrier coverage on it, where the band gap is around 20 meV. Furthermore, a simplified tight binding model based on the in-plane ddσ antibonding states is constructed to understand the topological band structures of the system.