The Fermi surface and band folding in La2-xSrxCuO4, probed by angle-resolved photoemission

A systematic angle-resolved photoemission study of the electronic structure of La2-xSrxCuO4 in a wide doping range is presented in this paper. In addition to the main energy band, we observed a weaker additional band, the (pi, pi) folded band, which shows unusual doping dependence. The appearance of the folded band suggests that a Fermi surface reconstruction is doping dependent and could already occur at zero magnetic field.

Doping dependence of the Fermi surface in the cuprates: A photoemission investigation

Helmuth Berger, Marco Grioni

We have exploited angle-resolved photoelectron spectroscopy (ARPES) to study the electronic structure of single crystal samples of high T-c material Bi2Sr2CaCu2O8+delta and of the antiferromagnetic insulator Sr2CuO2Cl2. As one moves away from optimal doping into underdoped region (T-c as low as 30K), the large Fermi surface sheet centered around the (pi,pi) point evolves into a small pocket centered around (pi/2,pi/2), and the saddle-point van Hove singularity is lost This evolution can be extrapolated to Sr2CuO2Cl2, which is apparently well described by the mean field spin-density wave calculation.

1996

Spectroscopic Studies on Iron-Chalcogenide Superconductors

Ping-Hui Lin

Superconductivity in F-doped LaFeAsO, Tc= 26 K was first reported by Kamihara et al in 2008. Following that, more iron-based superconductors, e.g., (Ba,K)Fe2As2, LiFeAs, and Fe1+yTe1-xSex were discovered. The new class of high Tc superconductors have attracted a tremendous interest. Iron-chacogenides do not involve arsenic but share several common features with the iron-pnictides. They are characterized by FeCh layers which are isostructural to the FeAs or FeP layers in the iron-pnictides. The electronic structure near the Fermi energy derives from the Fe 3d orbitals. Both families exhibit a spin density wave (SDW) ground state. Se substation for Te in FeTe suppresses the SDW and leads to superconductivity. From a more practical point of view, the iron-chacogenides have a rather simple structure, and they are less toxic and easier to obtain in large single crystals than the arsenide counterparts. For all these properties, the iron-chalcogenides are ideal compounds for angle-resolved photoemission (ARPES) studies. In this thesis, I have investigated by ARPES the normal state electronic structure of Fe1+yTe1-xSex superconductors with various doping level. I have studied the multi-orbital Fermi surface of Fe1+yTe1-xSex, mainly by means of polarized photons from synchrotron radiation sources. Individual bands of different orbital characters could then be distinguished by exploiting ARPES selection rules. I have also investigated the 3D electronic structure of these materials by performing normal emission measurements as a function of the incident photon energy. Namely, by using photon energies as large as 150 eV, I have performed an unusually broad ARPES survey of momentum space. The data reveal two inequivalent Fermi contours at nominally equivalent Gamma points in different Brillouin zones. The observed periodicity corresponds to a unit cell containing only one Fe atom, rather than the standard tetragonal unit cell containing two Fe atoms. I also observe a strong and selective suppression of spectral weight around perpendicular lines at the corner of the Brillouin zone.

EPFL2011

From hidden order to antiferromagnetism: Electronic structure changes in Fe-doped URu2Si2

Ji Dai, Emmanouil Frantzeskakis, Kevin Huang

In matter, any spontaneous symmetry breaking induces a phase transition characterized by an order parameter, such as the magnetization vector in ferromagnets, or a macroscopic many electron wave function in superconductors. Phase transitions with unknown order parameter are rare but extremely appealing, as they may lead to novel physics. An emblematic and still unsolved example is the transition of the heavy fermion compound URu2Si2 (URS) into the so-called hidden-order (HO) phase when the temperature drops below T-0 = 17.5 K. Here, we show that the interaction between the heavy fermion and the conduction band states near the Fermi level has a key role in the emergence of the HO phase. Using angle-resolved photoemission spectroscopy, we find that while the Fermi surfaces of the HO and of a neighboring antiferromagnetic (AFM) phase of well-defined order parameter have the same topography, they differ in the size of some, but not all, of their electron pockets. Such a nonrigid change of the electronic structure indicates that a change in the interaction strength between states near the Fermi level is a crucial ingredient for the HO to AFM phase transition.

NATL ACAD SCIENCES2021

Spectroscopic Studies on Iron-Chalcogenide Superconductors

Ping-Hui Lin

EPFL2011