U(1) LATTICE GAUGE MODEL FOR UNCONVENTIONAL SUPERCONDUCTORS: LINK COOPER PAIR AS DUAL GAUGE FIELD

Pressure-induced superconductivity extending across the topological phase transition in thallium-based topological materials

Classic and recent results for gauge effects on the properties of the normalto-superconducting phase transition in bulk and thin film superconductors are reviewed. Similar problems in the description of other natural systems (liquid crystals, quantum field theory, early universe) are also discussed. The relatively strong gauge effects on the fluctuations of the ordering field at low spatial dimensionality D and, in particular, in thin (quasi-2D) films are considered in details. A special attention is paid to the fluctuations of the gauge field. It is shown that the mechanism in which these gauge fluctuations affect on the order of the phase transition and other phase transition properties varies with the variation of the spatial dimensionality D. The problem for the experimental confirmation of the theoretical predictions about the order of the phase transitions in gauge systems is discussed.

The crystal is represented in the form of individual molecule sublattices. Within oscillation electronic model, a superconducting phase transition in an ABCD crystal is possible when the square plasma energy of the resulting molecular sublattices is equal (or larger) to the square plasma energy of the initial crystal. Lanthanum cuprate crystal is La2CuO4 considered. An elementary cell with two molecules z = 2 is considered, number of sublattices is n = 4. The square plasma energies of the initial crystal A B C D and its individual molecules A2, B2, C2, D2 were calculated. The phase transition curve of a superconductor is described by the quadratic function equation, where Tc is the temperature of the superconducting phase transition, q is the interaction parameter. From the equation of the phase transition curve in pure ideal mixed oxides of lanthanum and copper, a superconducting phase transition is not detected. When Lanthanum La is replaced by Strontium Sr, the superconducting transition temperature Tc reaches a value of up to 40K according to the literature data. Interaction parameter and order parameter are given. The influence of zinc and nickel impurities on the superconductivity of mixed copper and lanthanum oxides is calculated. The superconducting phase transition in a crystal proceeds as follows antiferromagnetic state - spin glass - phase separation - superconducting state. It should be noted that when the transition to the superconducting state of the base crystal, the dopants pass into the antiferromagnetic state. Superconducting - antiferromagnetic - paramagnetic state La, Superconducting - antiferromagnetic - paramagnetic - diamagnetic state Cu, Superconducting - antiferromagnetic - paramagnetic state Sr, Superconducting - antiferromagnetic - paramagnetic - diamagnetic state Zn, Superconducting - antiferromagnetic - paramagnetic - ferromagnetic state Nι. When calculating the temperature of the superconducting transition Tc for a crystal, it is necessary to consider the balance criterion of the square plasma energies, established for each temperature with allowance for thermodynamic equilibrium. Doping causes a violation of thermodynamic equilibrium at a certain temperature. This leads to the formation of valence bonds ―Cu ― О― with the release of energy with decreasing temperature, then a balance of energies is observed at a lower temperature, valence bonds are formed ―Cu ― Cu―, the released energy is spent on breaking the bonds ―Cu ― O― and so on.

The authors report resistivity measurements along the high-conductivity axis of (TMTSF)2ClO4, (TMTSF)2TaF6 and (TMTSF)2SbF6 for various hydrostatic pressures up to 12.5 kbar, and for transverse magnetic fields of up to 50 kG, in the temperature range 100 mK-4K. These measurements show that all three compounds have a superconducting phase transition near 1K when sufficient pressure is applied (TMTSF)2SbF6 and (TMTSF)2TaF6 become superconducting for pressures above 10 kbar and 11 kbar respectively. At lower pressures a metal-insulator phase transition is observed which is probably associated with a spin density wave (SDW) instability. They observe a superconducting phase transition in (TMTSF)2ClO4 at much lower pressures (<3 kbar) and at 1.5 kbar there is a minimum in the resistivity above the superconducting phase transition temperature indicating the possible coexistence of the SDW and superconducting phases. They show that the critical pressure above which the superconducting phase is stabilised can be correlated with the separation between the sheets of TMTSF molecules and anions and discuss possible implications with reference to all members of the family of (TMTSF)2X salts.

We briefly review the nonlinear sigma model approach for the subject of increasing interest: "two-step" phase transitions in the Gross–Neveu and the modified Nambu–Jona-Lasinio models at low N and condensation from pseudogap phase in strong-coupling superconductors. Recent success in describing of "Bose-type" superconductors that possess two characteristic temperatures and a pseudogap above Tc is the development approximately comparable with the BCS theory. One can expect that it should have influence on high-energy physics, similar to impact of the BCS theory on this subject. Although first generalizations of this concept to particle physics were made recently, these results were not systematized. In this review we summarize this development and discuss similarities and differences of the appearance of the pseudogap phase in superconductors and the Gross–Neveu and Nambu–Jona-Lasinio-like models. We discuss its possible relevance for chiral phase transition in QCD and color superconductors. This paper is organized in three parts. In the first part, we briefly review the separation of temperatures of pair formation and pair condensation in strong-coupling and low carrier density superconductors (i.e. the formation of the pseudogap phase). The second part is a review of nonlinear sigma model approach to an analogous phenomenon in the chiral Gross–Neveu model at small N. In the third part we discuss the modified Nambu–Jona-Lasinio model where the chiral phase transition is accompanied by a formation of a phase analogous to the pseudogap phase.

Pressure-induced superconductivity and quantum phase transitions in the Rashba material BiTeCl

We present a non-perturbative theory of paramagnetically-driven superconducting (SC) phase transitions in heavy-fermion metals, which reliably determines their stable SC phases, treats properly the corresponding finite jumps of the order parameter, and can account for unusual features reported recently for this type of materials. It is found that for quasi-2D heavy-fermion metals, such as CeCoIn5, at high magnetic fields oriented perpendicular to the highly conducting planes, the effect of the Fulde-Ferrel (FF) modulation is too weak to prevent a direct first-order phase transition from the normal to the uniform SC state. For 3D heavy-fermion metals, such as URu2Si2, the FF modulation stabilizes, under a decreasing magnetic field, a non-uniform SC state via a second-order phase transition from the normal state. However, at a slightly lower field the modulated phase becomes unstable, transforming to a uniform SC state via a first-order transition.

Theory of quantum phase transition in iron-based superconductors with half-Dirac nodal electron Fermi surface

Exotic magnetic and superconducting phases in CeCu 2Si 2 close to magnetically ordered state : Cu-NQR study

Recent experimental advances in atomically thin transition metal dichalcogenide (TMD) metals have unveiled a range of interesting phenomena including the coexistence of charge-density-wave (CDW) order and superconductivity down to the monolayer limit. The atomic thickness of two-dimensional (2D) TMD metals also opens up the possibility for control of these electronic phase transitions by electrostatic gating. Here, we demonstrate reversible tuning of superconductivity and CDW order in model 2D TMD metal NbSe_{2} by an ionic liquid gate. A variation up to ∼50% in the superconducting transition temperature has been observed. Both superconductivity and CDW order can be strengthened (weakened) by increasing (reducing) the carrier density in 2D NbSe_{2}. The doping dependence of these phase transitions can be understood as driven by a varying electron-phonon coupling strength induced by the gate-modulated carrier density and the electronic density of states near the Fermi surface.

Effect of pressure on superconductivity and structural phase transitions in La[2-x]M[x]CuO[4] (M=Ba, Sr)

Quantum phase transition in lattice model of unconventional superconductors

Quantum criticality and superconductivity in quasi-two-dimensional Dirac electronic systems

Enhancement of superconductivity and structural phase transitions under pressure in FeTe0.89S0.11

The question whether the Halperin-Lubensky-Ma result for a fluctuation-induced weakly first-order phase transition in superconductors holds in the presence of quenched impurities is considered. The renormalisation-group recursion relations have a new stable fixed point for 1<n<or=366, which describes a real critical behaviour in the range 2<Dc(n)<d<4 of space dimensionalities d (n/2 is the number of components of the complex order parameter). Some features of the new fixed point are discussed. The critical exponents are presented for 1<n<or=366.