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Register a new userQuasiparticle interference of heavy fermions in resonant X-ray scattering
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Resonant X-ray scattering (RXS) has recently become an increasingly important tool for the study of ordering phenomena in correlated electron systems. Yet, the interpretation of the RXS experiments remains theoretically challenging due to the complexity of the RXS cross-section. Central to this debate is the recent proposal that impurity-induced Friedel oscillations, akin to quasiparticle interference signals observed with the scanning tunneling microscope (STM), can lead to scattering peaks in the RXS experiments. The possibility that quasiparticle properties can be probed in RXS measurements opens up a new avenue to study the bulk band structure of materials with the orbital and element selectivity provided by RXS. Here, we test these ideas by combining RXS and STM measurements of the heavy fermion compound CeMIn$_5$ (M = Co, Rh). Temperature and doping dependent RXS measurements at the Ce-M$_4$ edge show a broad scattering enhancement that correlates with the appearance of heavy f-electron bands in these compounds. The scattering enhancement is consistent with the measured quasiparticle interference signal in the STM measurements, indicating that quasiparticle interference can be probed through the momentum distribution of RXS signals. Overall, our experiments demonstrate new opportunities for studies of correlated electronic systems using the RXS technique.
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To fully capitalize on the potential and versatility of resonant inelastic x-ray scattering (RIXS), it is essential to develop the capability to interpret different RIXS contributions through calculations, including the dependence on momentum transfer, from first-principles for correlated materials. Toward that objective, we present new methodology for calculating the full RIXS response of a correlated metal in an unbiased fashion. Through comparison of measurements and calculations that tune the incident photon energy over a wide portion of the Fe L$_3$ absorption resonance of the example material BaFe$_2$As$_2$, we show that the RIXS response in BaFe$_2$As$_2$ is dominated by the direct channel contribution, including the Raman-like response below threshold, which we explain as a consequence of the finite core-hole lifetime broadening. Calculations are initially performed within the first-principles Bethe-Salpeter framework, which we then significantly improve by convolution with an effective spectral function for the intermediate-state excitation. We construct this spectral function, also from first-principles, by employing the cumulant expansion of the Greens function and performing a real-time time dependent density functional theory calculation of the response of the electronic system to the perturbation of the intermediate-state excitation. Importantly, this allows us to evaluate the indirect RIXS response from first-principles, accounting for the full periodicity of the crystal structure and with dependence on the momentum transfer.
We report a resonant x-ray scattering (RXS) study of antiferromagnetic neptunium compounds NpCoGa_5 and NpRhGa_5 at the Np M_4 and Ga K-edges. Large resonant signals of magnetic dipole character are observed below the Neel temperatures at both edges. The signals at the Np edges confirm the behaviour determined previously from neutron diffraction, i.e. the moments along [001] in NpCoGa_{5} and in NpRhGa_5 a reorientation of the moments from the c-axis direction to the ab plane. In the latter material, on application of magnetic field of 9 Tesla along the [010] direction we observe a change in the population of different [110]-type domains. We observe also a magnetic dipole signal at the Ga K-edge, similarly to the reported UGa_3 case, that can be interpreted within a semi-localized model as an orbital polarization of the Ga 4p states induced via strong hybridization with the Np 5f valence band. Quantitative analysis of the signal shows that the Ga dipole on the two different Ga sites follows closely the Np magnetic moment reorientation in NpRhGa_5. The ratios of the signals on the two inequivalent Ga sites are not the same for the different compounds.
Comprehensive x-ray scattering studies, including resonant scattering at Mn L-edge, Tb L- and M-edges, were performed on single crystals of TbMn2O5. X-ray intensities were observed at a forbidden Bragg position in the ferroelectric phases, in addition to the lattice and the magnetic modulation peaks. Temperature dependences of their intensities and the relation between the modulation wave vectors provide direct evidences of exchange striction induced ferroelectricity. Resonant x-ray scattering results demonstrate the presence of multiple magnetic orders by exhibiting their different temperature dependences. The commensurate-to-incommensurate phase transition around 24 K is attributed to discommensuration through phase slipping of the magnetic orders in spin frustrated geometries. We proposed that the low temperature incommensurate phase consists of the commensurate magnetic domains separated by anti-phase domain walls which reduce spontaneous polarizations abruptly at the transition.
We consider a two-orbital tight-binding model defined on a layered three-dimensional hexagonal lattice to investigate the properties of topological nodal lines and their associated drumhead surface states. We examine these surface states in centrosymmetric systems, where the bulk nodal lines are of Dirac type (i.e., four-fold degenerate), as well as in non-centrosymmetric systems with strong Rashba and/or Dresselhaus spin-orbit coupling, where the bulk nodal lines are of Weyl type (i.e., two-fold degenerate). We find that in non-centrosymmetric systems the nodal lines and their corresponding drumhead surface states are fully spin polarized due to spin-orbit coupling. We show that unique signatures of the topologically nontrivial drumhead surface states can be measured by means of quasiparticle scattering interference, which we compute for both Dirac and Weyl nodal line semimetals. At the end, we analyze the possible crystal structures with a symmetry that supports flat surface states which are effectively ringlike.
We have performed a resonant x-ray scattering (RXS) study near the Co K edge on a single crystal of Ca3Co2O6. In the magnetically ordered phase a new class of weak reflections appears at the magnetic propagation vector tau (1/3,1/3,1/3). These new reflections allow direct access to the dipolar-quadrupolar E1E2 scattering channel. The theoretical possibility of observing isolated E1E2 electromagnetic multipoles has attracted a lot of interest in the recent years. Unfortunately in many system of interest, parity even and parity odd tensor contributions occur at the same positions in reciprocal space. We demonstrate that in Ca3Co2O6 it is possible to completely separate the parity even from the parity odd terms. The possibility of observing such terms even in globally centrosymmetric systems using RXS has been investigated theoretically; Ca3Co2O6 allows a symmetry based separation of this contribution.
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