Unconventional superconductor

Summary

Unconventional superconductors are materials that display superconductivity which does not conform to conventional BCS theory or its extensions. The superconducting properties of CeCu2Si2, a type of heavy fermion material, were reported in 1979 by Frank Steglich. For a long time it was believed that CeCu2Si2 was a singlet d-wave superconductor, but since the mid 2010s, this notion has been strongly contested. In the early eighties, many more unconventional, heavy fermion superconductors were discovered, including UBe13, UPt3 and URu2Si2. In each of these materials, the anisotropic nature of the pairing was implicated by the power-law dependence of the nuclear magnetic resonance (NMR) relaxation rate and specific heat capacity on temperature. The presence of nodes in the superconducting gap of UPt3 was confirmed in 1986 from the polarization dependence of the ultrasound attenuation. The first unconventional triplet superconductor, organic material (TMTSF)2PF6, was discovered by Denis Jerome, Klaus Bechgaard and coworkers in 1980. Experimental works by Paul Chaikin's and Michael Naughton's groups as well as theoretical analysis of their data by Andrei Lebed have firmly confirmed unconventional nature of superconducting pairing in (TMTSF)2X (X=PF6, ClO4, etc.) organic materials. High-temperature singlet d-wave superconductivity was discovered by J.G. Bednorz and K.A. Müller in 1986, who also discovered that the lanthanum-based cuprate perovskite material LaBaCuO4 develops superconductivity at a critical temperature (Tc) of approximately 35 K (-238 degrees Celsius). This was well above the highest critical temperature known at the time (Tc = 23 K), and thus the new family of materials was called high-temperature superconductors. Bednorz and Müller received the Nobel prize in Physics for this discovery in 1987. Since then, many other high-temperature superconductors have been synthesized. LSCO (La2−xSrxCuO4) was discovered the same year (1986).

Official source

https://en.wikipedia.org/wiki/Unconventional_superconductor

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Probing the reconstructed Fermi surface of antiferromagnetic BaFe2As2 in one domain

Timur Kim

A fundamental part of the puzzle of unconventional superconductivity in the Fe-based superconductors is the understanding of the magnetic and nematic instabilities of the parent compounds. The issues of which of these can be considered the leading instability, and whether weak- or strong-coupling approaches are applicable, are both critical and contentious. Here, we revisit the electronic structure of BaFe2As2 using angle-resolved photoemission spectroscopy (ARPES). Our high-resolution measurements of samples "detwinned" by the application of a mechanical strain reveal a highly anisotropic 3D Fermi surface in the low-temperature antiferromagnetic phase. By comparison of the observed dispersions with ab initio calculations, we argue that overall it is magnetism, rather than orbital/nematic ordering, which is the dominant effect, reconstructing the electronic structure across the Fe 3d bandwidth. Finally, using a state-of-the-art nano-ARPES system, we reveal how the observed electronic dispersions vary in real space as the beam spot crosses domain boundaries in an unstrained sample, enabling the measurement of ARPES data from within single antiferromagnetic domains, and showing consistence with the effective mono-domain samples obtained by detwinning.

Springer2019

From recent advances in solid state physics, a novel material classification scheme has evolved which is based on the concept of topology and provides an understanding of different phenomena ranging from quantum transport to unusual flavors of superconductivity. Spin-orbit coupling is a major term in defining the topology of materials, and its interplay with electron electron correlations yields novel, intriguing phenomena. In the weak to intermediate correlation regime, spin-orbital entanglement leads to the emergence of topological insulators, which constitute Dirac materials with 2D spin-polarized helical edge or surface states and an insulating bulk. In the specific case of topological insulators, theory predicts that their unique band structure leads to a high thermoelectric performance, which is of practical relevance for energy conversion applications.In the strong correlation regime, the spin-orbit coupling removes the orbital degeneracy, thereby enhancing quantum fluctuations of the spin-orbit entangled states. This in turn can lead to the emergence of novel phases such as quantum spin liquids and unconventional superconductivity. The Kitaev quantum spin liquid (QSL) is a topological state of matter that exhibits fractionalized excitations in the form of Majorana fermions. The main aim of this thesis was to study these novel states of matter in the form of two different materials that fall in the weak and strong correlation regime, respectively. On this basis, it should furthermore be explored whether benefit can be taken of their properties by combining them with other quantum materials into heterostructures.In the first part of this thesis, nanoplatelets of bismuth chalcogenide-based 3D topological insulators were grown by chemical vapor deposition, and characterized by magnetotransport experiments. Having established charge transport characteristics of the single components, in the aim to exploit their high thermoelectric efficiency, the lateral heteroctructures of them are fabricated. Their investigation by scanning photocurrent microscopy revealed strong photocurrent generation at the p-n junction. The obtained results demonstrate that such type of hybrid heterostructure is a promising route to enhance the thermoelectric performance in nanostructured devices. In the second part of this thesis, the Kitaev spin liquid candidate, RuCl3 was studied. Raman spectroscopy on exfoliated RuCl3 revealed a broad magnetic continuumat low energies which according to its particular deviation from bosonic character is assigned to the fractionalized excitations. Furthermore, in complementary charge transport experiments, it was found that themagnetic fluctuations and structural changes in thismaterial are highly entangled, and influence the emergence of the fractionalized excitations. Finally, it was investigated whether the magnetic insulator character of ®-RuCl3 can be exploited to alter the graphene band structure through a proximity effect in vertical graphene/ RuCl3 heterostructures. Measurements of their low-temperature, nonlocal resistance confirmed the possibility to induce pure spin currents in this manner. At the same time, the charge transport through the graphene served as a probe of the magnetic ordering in RuCl3, although the complex magnetic behavior in this material necessitates more studies.

EPFL2018

In materials where electrons interact strongly, a number of exotic and exciting phenomena arise. The mechanisms at the base of many of these phenomena remain debated, as strongly correlated electron physics represents one of the biggest challenges for modern condensed matter physics. Transition metal oxides are a class of strongly correlated systems which exhibit a multitude of physical properties, among which unconventional superconductivity, incommensurate charge and spin ordering, partial anisotropic gapping, strangemetallicity, colossal magnetoresistance, multiferroicity, antiferromagneticMott insulation, etc.. These properties are often observed in close vicinity, when pressure, temperature or chemical composition are varied. As a consequence, it is challenging to disentangle not only their origin, but also their effects on the elementary excitations of the system. In this thesis, high quality single crystals of copper and ruthenium based transition metal oxides (cuprate and ruthenate compounds) were investigated with synchrotron-based spectroscopic techniques. Our angle resolved photoemission spectroscopy (ARPES) studies focused on the normal state of cuprates, at hole content varying from under- to over-doping. First, the low energy single particle excitations in overdoped uprates were verified to fulfill the mathematical conditions for Landau Fermi quasiparticles. In a second study, the evolution of the spectral gap was followed as function of doping and temperature, across the charge order, pseudogap and strangcuprates, ruthenates, strong correlations, RIXS, ARPES, XAS, pseudogap, strange metal, spin orbit coupling, crystal field.e metal phases of a cuprate compound, La1.6¡xNd0.4SrxCuO4. This systematic study allowed the identification of an optimal doping regime for the investigation of the pseudogap physics in cuprates. The orbital structure of single layer ruthenates was explored combining X-ray absorption and resonant inelastic X-ray spectroscopies (XAS and RIXS). Since spectroscopies at the L absorption edge of 4d materials are challenged by insufficient energy resolution, we performed our studies at the oxygen K edge, in the soft X-ray energy range. By exploiting the strong orbital hybridization between the oxygen 2p and the ruthenium 4d states, we obtained high resolution experimental access to the 4d electron properties. The results were interpreted through a simple model Hamiltonian, with analytic solutions that provide a consistent description of the low energy features observed.

EPFL2017

EPFL2018

EPFL2017