Strongly correlated material | EPFL Graph Search

Strongly correlated materials are a wide class of compounds that include insulators and electronic materials, and show unusual (often technologically useful) electronic and magnetic properties, such as metal-insulator transitions, heavy fermion behavior, half-metallicity, and spin-charge separation. The essential feature that defines these materials is that the behavior of their electrons or spinons cannot be described effectively in terms of non-interacting entities. Theoretical models of the electronic (fermionic) structure of strongly correlated materials must include electronic (fermionic) correlation to be accurate. As of recently, the label quantum materials is also used to refer to strongly correlated materials, among others. Transition metal oxides Many transition metal oxides belong to this class which may be subdivided according to their behavior, e.g. high-Tc, spintronic materials, multiferroics, Mott insulators, spin Peierls materials, heavy fermion materials, q

Resonant Inelastic X-ray Scattering Studies of Low-Dimensional and Frustrated 3d Transition-Metal Oxides

Yi Tseng

3d transition-metal oxide materials with strong electron correlations and low dimensionality give rise to emerging exotic phases. Resonant inelastic X-ray scattering (RIXS) has developed as a powerful spectroscopic tool for probing the collective excitations in such correlated materials. The RIXS technique provides an ideal approach for assessing electronic instabilities in condensed matter. This thesis comprises the experimental RIXS response of the spin chain-ladder cuprates Sr14-xCax(Cu,Co)24O41 and frustrated honeycomb nickelate Na2Ni2TeO6. The spin and charge excitations of Sr14-xCaxCu24O41 (x=0 & 12.2) are studied by Cu L3-edge RIXS. With increasing Ca content, a crossover from collective two-triplon excitations (x=0) to a damped incoherent magnetic spectrum ~280 meV (x=12.2) in the ladders is uncovered from the RIXS spectra, dominated by spin-flip ÎS=1 scattering. With model calculations, the localized broad magnetic mode at x=12.2 is shown to be a consequence of enhanced elecrtronic localization. This is supported by polarization-dependent measurements, where the evaluated non-spin-flip ÎS=0 scattering is showing weight depletion below 1 eV. The RIXS spectral evolution suggests that a suppressed carrier mobility dominates the low-energy physics in Ca-rich phases. The low-energy excitations of Sr14Cu24O41 are also studied by O K-edge RIXS at the upper Hubbard band. The RIXS spectra show a sharp dispersing peak of similar energies to the ÎS=1 two-triplon excitations, and a broad non-dispersive mode at higher-energy. By comparing the experimental results to the existing theories of doped ladders, the observed spectral modes are attributed to interacting holon-spinon quasiparticles, with additional spectral contributions from the ÎS=0 multi-triplon bound states and continuum. These observations highlight the RIXS capability for resolving composite spin-charge quasiparticles and long-lived ÎS=0 magnetic excitations, which are difficult to detect by other experiments. Charge order and phonons for the two-leg ladders of Sr14(Cu,Co)24O41 are studied. At the ladder hole peak in O K-edge XAS signal, a concomitant elastic enhancement and phonon-softening is observed across all Co doping levels. The in-plane diffraction and phonon-softening response are found to enhance with increasing Co doping. Morevoer, the observed diffraction signal is further characterized by energy- and temperature-dependent measurements. Lastly, given the strong electron-phonon coupling (EPC) observed in O K-edge RIXS spectra, the EPC strength can be analyzed from the RIXS intensity decay of the phonon overtones . The observed in-plane resonant diffraction and inelastic phonon response strongly suggest an enhanced stripe order via magnetic impurities, reminiscent of the reports in CuO4 plaquettes. Electronic structure and EPC of the honeycomb nickelate Na2Ni2TeO6 are studied. Compared with current literature, the localized excitations (1-4 eV) in the Ni L3-edge RIXS spectra are dominated by the crystal-field splitting of a Ni2+ ion in octahedral coordination. At the O K-edge, the large EPC ~285 meV with 5 recognizable overtones (frequency of ~80 meV) are revealed in the RIXS spectra. The observed modes provide a systematic characterization of charge, orbital and lattice dynamics for a newly proposed frustrated antiferromagnet. This thesis has been carried out in a collaboration between the Paul Scherrer Institut and the Laboratory for Quantum Magnetism at EPFL.

EPFL2021

Semiclassical methods in conformal field theories scrutinized by the epsilon-expansion

Gil Badel

Conformal Field Theories (CFTs) are crucial for our understanding of Quantum Field Theory (QFT). Because of their powerful symmetry properties, they play the role of signposts in the space of QFTs. Any method that gives us information about their structure, and lets us compute their observables, is therefore of great interest. In this thesis we explore the large quantum number sector of CFTs, by describing a semiclassical expansion approach. The idea is to describe the theory in terms of fluctuations around a classical background, which corresponds to a superfluid state of finite charge density. We detail the implementation of the method in the case of U (1)-invariant lagrangian CFTs defined in the epsilon-expansion. After introducing the method for generic correlators, we illustrate it by performing the computation of several observables.First, we compute the scaling dimension of the lowest operator having a given large charge n under the U (1) symmetry. We demonstrate how the semiclassical result in this case bridges the gap between the naive diagrammatic computation (which fails at too large n) and the general large-charge expansion of CFTs (which is only valid for n large enough).Second, we apply the method to the computation of 3- and 4-point functions involving the same operator. This lets us derive some of the OPE (Operator Product Expansion) coefficients.Finally, we consider the rest of the spectrum of charge-n operators, and propose a way to classify them by studying their free-theory equivalent. In the free theory, we construct the complete set of primary operators with number of derivatives bounded by the charge.We also find a mapping between the excited states of the superfluid and the vacuum states of standard quantization, which is valid when the spin of said states is bounded by the square root of the charge.

EPFL2022

Direct observation of the spin texture in SmB6 as evidence of the topological Kondo insulator

Kazimierz Conder, Jan Hugo Dil, Joël Mesot, Stefan Peter Muff

Topological Kondo insulators have been proposed as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to strong spin-orbit coupling. In contrast to other three-dimensional topological insulators, a topological Kondo insulator is truly bulk insulating. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. By applying spin- and angle-resolved photoemission spectroscopy, here we show that the surface states of SmB6 are spin polarized. The spin is locked to the crystal momentum, fulfilling time reversal and crystal symmetries. Our results provide strong evidence that SmB6 can host topological surface states in a bulk insulating gap stemming from the Kondo effect, which can serve as an ideal platform for investigating of the interplay between novel topological quantum states with emergent effects and competing orders induced by strongly correlated electrons.

Nature Publishing Group2014

Resonant Inelastic X-ray Scattering Studies of Low-Dimensional and Frustrated 3d Transition-Metal Oxides

Yi Tseng

EPFL2021