Close to a zero-temperature transition between ordered and disordered electronic phases, quantum fluctuations can lead to a strong enhancement of electron mass and to the emergence of competing phases such as superconductivity. A correlation between the existence of such a quantum phase transition and superconductivity is quite well established in some heavy fermion and iron-based superconducto...

Recent development in the physics of high-temperature superconductivity is reviewed, with special emphasis on the studies of the low-energy excitations of cuprate and iron-based superconductors. For cuprate superconductors, a phenomenology based on coexisting competing orders with superconductivity in the ground state of these doped Mott insulators is shown to provide a consistent account for a...

A major problem in the field of high-transition temperature (Tc) superconductivity is the identification of the electronic instabilities near superconductivity. It is known that the iron-based superconductors exhibit antiferromagnetic order, which competes with the superconductivity. However, in the nonmagnetic state, there are many aspects of the electronic instabilities that remain unclarifie...

Superconductivity requires a “pairing interaction”—an indirect attractive force between the conduction electrons that can overcome their direct Coulomb repulsion. To prove which particular degrees of freedom provide this pairing interaction is supremely difficult: nearly 40 years elapsed between the discovery of superconductivity in 1911 and the “smoking gun” measurement of the isotope effect t...

Topological superconductivity is a state of matter that can host Majorana modes, the building blocks of a topological quantum computer. Many experimental platforms predicted to show such a topological state rely on proximity-induced superconductivity. However, accessing the topological properties requires an induced hard superconducting gap, which is challenging to achieve for most material sys...

Charge and spin stripe order is a type of electronic crystal observed in certain layered cuprates associated with high-temperature superconductivity. Quantum-disordered stripes could be relevant for understanding the superconductivity. Here I discuss recent experimental characterizations of the stripe-ordered state in La1.875Ba0.125CuO4, and compare them with properties of superconducting compo...

Superconductivity up to 43 K and magnetic ordering coexist in the iron chalcogenides [(Li(0.8)Fe(0.2))OH]Fe(S(1-x)Se(x)) (0 < x ≤ 1). Substitution of sulphur for selenium gradually suppresses superconductivity while the ferromagnetic signature persists up to non-superconducting [(Li(0.8)Fe(0.2))OH]FeS.

Following Kamerlingh Onnes’ discovery of zero resistance, it took a very long time to understand how superconducting electrons can move without hindrance through a metal. Attempts to explain from first principles how superconductivity comes about proved to be one of the most intractable problems of physics. Progress required more than just new data; it needed an innovative theoretical framework...

s of Doctor′s Theses (Received The Degree of Doctorof Engineering in 2006) ..................................................................................................................................................................... 24 Abstracts of Papers (Published in 2006) ....................................................................................................................

According to quantum chromodynamics, matter at ultra-high density and low temperature is a quark liquid, with a condensate of Cooper pairs of quarks near the Fermi surface (“color superconductivity”). This paper reviews the physics of color superconductivity, and discusses some of the proposed signatures by which we might detect quark matter in neutron stars.