Shantanu Mukherjee

Discovery of orbital-selective Cooper pairing in FeSe

A deeper look into iron selenide In the past 10 years, iron-based superconductors have created more puzzles than they have helped resolve. Some of the most fundamental outstanding questions are how strong the interactions are and what the electron pairing mechanism is. Now two groups have made contributions toward resolving these questions in the intriguing compound iron selenide (FeSe) (see the Perspective by Lee). Gerber et al. used photoemission spectroscopy coupled with x-ray diffraction to find that FeSe has a very sizable electron-phonon interaction. Quasiparticle interference imaging helped Sprau et al. determine the shape of the superconducting gap and find that the electron pairing in FeSe is orbital-selective. Science, this issue p. 71, p. 75; see also p. 32 Cooper pairing in iron selenide predominantly occurs between electrons from dyz orbitals of iron atoms. The superconductor iron selenide (FeSe) is of intense interest owing to its unusual nonmagnetic nematic state and potential for high-temperature superconductivity. But its Cooper pairing mechanism has not been determined. We used Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry of the electronic bands surrounding the Γ = (0, 0) and X = (π/aFe, 0) points of FeSe and to measure the corresponding superconducting energy gaps. We show that both gaps are extremely anisotropic but nodeless and that they exhibit gap maxima oriented orthogonally in momentum space. Moreover, by implementing a novel technique, we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbital-selective Cooper pairing that, in FeSe, is based preferentially on electrons from the dyz orbitals of the iron atoms.

Low-temperature ferroelectric phase and magnetoelectric coupling in underdoped La2CuO4+x

We report the discovery of ferroelectricity below 4.5 K in highly underdoped La2CuO4+x accompanied by slow charge dynamics which develop below T similar to 40 K. An anisotropic magnetoelectric response has also been observed, indicating considerable spin-charge coupling in this lightly doped parent high-temperature copper-oxide superconductor. The ferroelectric state is proposed to develop from polar nanoregions, in which spatial inversion symmetry is locally broken due to nonstoichiometric carrier doping.

Ferroelectricity in underdoped La-based cuprates

Doping a “parent” antiferromagnetic Mott insulator in cuprates leads to short-range electronic correlations and eventually to high-Tc superconductivity. However, the nature of charge correlations in the lightly doped cuprates remains unclear. Understanding the intermediate electronic phase in the phase diagram (between the parent insulator and the high-Tc superconductor) is expected to elucidate the complexity both inside and outside the superconducting dome, and in particular in the underdoped region. One such phase is ferroelectricity whose origin and relation to the properties of high-Tc superconductors is subject of current research. Here we demonstrate that ferroelectricity and the associated magnetoelectric coupling are in fact common in La-214 cuprates namely, La2-xSrxCuO4, La2LixCu1-xO4 and La2CuO4+x. It is proposed that ferroelectricity may result from local CuO6 octahedral distortions, associated with the dopant atoms and clustering of the doped charge carriers, which break spatial inversion symmetry at the local scale whereas magnetoelectric coupling can be tuned through Dzyaloshinskii-Moriya interaction.

Theory of the magnetoeletric effect in a lightly doped high-Tc cuprate

In a recent study, Viskadourakis et al. (arXiv:1111.0050) discovered that extremely underdoped La

Resonant plasmon-axion excitations induced by charge density wave order in a Weyl semimetal

{}_{2}

Bi-quadratic magnetoelectric coupling in underdoped La2CuO4+x

CuO

Tuning a strain-induced orbital selective Mott transition in epitaxialVO2

{}_{4+x}

Structural and magnetic field effects on spin fluctuations in Sr3Ru2O7

is a relaxor ferroelectric and a magnetoelectric material at low temperatures. It is further observed that the magnetoelectric response is anisotropic for different directions of electric polarization and applied magnetic field. By constructing an appropriate Landau theory, we show that a biquadratic magnetoelectric coupling can explain the experimentally observed polarization dependence on magnetic field. This coupling leads to several interesting low-temperature effects, including a feedback enhancement of the magnetization within the ferroelectric phase, and a predicted magnetocapacitive effect.

Low-energy bound states at interfaces between superconducting and block antiferromagnetic regions in KxFe2-ySe2

We investigate the charge excitations of a Weyl semimetal in the axionic charge density wave (axionic CDW) state. While it has been shown that the topological response (anomalous Hall conductivity) is protected against the CDW state, we find that the long wavelength plasmon excitation is radically influenced by the dynamics of the CDW order parameter. In the normal state, we show that an undamped collective mode should exist at q~Q_(CDW) if there is an attractive interaction favoring the formation of the CDW state. The undamped nature of this collective mode is attributed to a gap-like feature in the particle-hole continuum at q~Q_(CDW) due to the chirality of the Weyl nodes, which is not seen in other materials with CDW instability. In the CDW state, the long wavelength plasmon excitations become more dispersive due to the additional interband scattering not allowed in the normal state. Moreover, because the translational symmetry is spontaneously broken, Umklapp scattering, the process conserving the total momentum only up to nQ_(CDW) with n an integer and Q_(CDW) the ordering wave vector, emerges in the CDW state. We find that the plasmon excitation couples to the phonon mode of the CDW order via the Umklapp scattering, leading to two branches of resonant collective modes observable in the density-density correlation function at q~0 and q~Q_(CDW). Based on our analysis, we propose that measuring these resonant plasmon-axion excitations around q~0 and q~Q_(CDW) by the momentum-resolved electron energy loss spectroscopy (M-EELS) could serve as a reliable way to detect the axionic CDW state in Weyl semimetals.

Impurity-induced subgap bound states in alkali-doped iron chalcogenide superconductors

The recent discovery of relaxor ferroelectricity and magnetoelectric effect in lightly doped cuprate material La2CuO4+x has provided a number of questions concerning its theoretical description. Using a Ginzburg–Landau free energy approach, it has been argued that the magnetoelectric effect can be explained by bi-quadratic interaction terms in the free energy. Here, by using the same free energy functional, we study the variety of behavior which can emerge in the electric polarization under an external magnetic field. Subsequently, we discuss the role of Dzyaloshinskii–Moriya interaction in generating this magnetoelectric response. This work is particularly relevant for such relaxor systems where the material-dependent parameters would be affected by changes in, e.g., chemical doping or cooling rate.