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Recently some experimental evidences have been obtained in favour of the existence of the inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state in heavy-fermion superconductor CeCoIn_{5} and organic superconductor…
The origin of the pseudogap behavior, found in many high-$T_c$ superconductors, remains one of the greatest puzzles in condensed matter physics. One possible mechanism is fermionic incoherence, which near a quantum critical point allows…
The importance of electron-hole interband interactions is widely acknowledged for iron-pnictide superconductors with high transition temperatures (Tc). However, high-Tc superconductivity without hole carriers has been suggested in FeSe…
It is discussed some problems of normal states of high temperature superconductors (HTS's) with d-wave pairing and next suppositions: 1) at pseudogap temperature it occurs the dimensional crossover from 3D incoherent normal state to quasi…
Superconductivity is a quantum phenomenon caused by bound pairs of electrons. In diverse families of strongly correlated electron systems, the electron pairs are not bound together by phonon exchange but instead by some other kind of…
In the generic phase diagram of heavy fermion systems, tuning an external parameter such as hydrostatic or chemical pressure modifies the superconducting transition temperature. The superconducting phase forms a dome in the…
The fluctuation conductivity of a moderately clean type II superconductor with strong Pauli paramagnetic pair-breaking (PPB) is studied by focusing on the quantum regime at low temperatures and in high magnetic fields. First, it is pointed…
We study the superconducting transition temperature, T_c, of an array of fluctuating, maximally gapped interacting ladders, embedded in a disordered plane. Renormalization group analysis indicates that geometrical fluctuations mitigate the…
Motivated by the superconductivity near paraelectric (PE) to ferroelectric (FE) quantum critical point (QCP) in polar metals, we study polar fluctuation mediated superconductivity in multi-orbital systems. The PE to FE QCP is approached by…
An effective mass of charge carriers that is significantly larger that the mass of a free electron develops at low temperatures in certain lanthanide- and actinide-based metals, including those formed with plutonium, due to strong…
We find new mechanism of superconductivity beyond the spin-fluctuation theory, the standard model for unconventional superconductivity in the weak coupling approach, where Kondo fluctuations result in multi-gap superconductivity around an…
In Uranium-based heavy fermion system, the 5f electrons display an intermediate character between partial localization and partial itinerancy, which makes the Kondo problem more complicated. Here we use scanning tunneling…
The microscopic mechanism of heavy band formation, relevant for unconventional superconductivity in CeCoIn$_5$ and other Ce-based heavy fermion materials, depends strongly on the efficiency with which $f$ electrons are delocalized from the…
The hole-doped cuprate high temperature superconductors enter the pseudogap regime as their superconducting critical temperature, $T_c$, falls with decreasing hole density. Experiments have probed this regime for over two decades, but we…
By combining experimental measurements of the quasiparticle and dynamical magnetic properties of optimally electron-doped Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$ with theoretical calculations we demonstrate that the conventional fermiology approach…
A fluctuating-valence impurity in a metal is quantum-critical unlike a Kondo impurity which has the properties of a local Fermi-liquid. A systematic theory for the fluctuating-valence lattice is constructed, based on the hybridization and…
We show that the low-energy fluctuations of the valence bond in underdoped high-T_c cuprates, originating from quantum fluctuations of the superexchange interaction, are pinned by the electronic disorder due to off-stoichiometric dopants,…
The electronic states and superconductivity in iron pnictides are studied on the basis of the 16 band $d$-$p$ model which includes both the onsite Coulomb interaction between Fe $d$ electrons and the intersite one between Fe $d$ and…
The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based…
We investigate the excitonic fluctuation and its mediated superconductivity in the quasi one-dimensional three-chain Hubbard model for Ta$_2$NiSe$_5$ known as a candidate material for the excitonic insulator. In the semiconducting case and…