L. P. Gor’kov

Manganites at low temperatures and light doping: band approach and percolation

A tight-band model is employed for thee2g orbitals in manganites. It is shown that a large intra-atomic Hund couplingJH and the resulting double-exchange mechanism lead to antiferromagnetic ordering along one of the cubic axes, stabilized by the cooperative Jahn-Teller effect, which further decreases the band energy, of the electrons. As a result, LaMnO3 is a band insulator built of 2D ferromagnetic layers. The critical concentration (xc ≃0.16) for the onset of ferromagnetic and metallic behavior at low temperatures in La1−xSrxMnO3 and the phase transition are treated in a percolation approach.

On superconducting and magnetic properties of iron oxypnictides

AbstractPairing symmetry in oxypnictides, a new family of multiband high-Tc superconductors, is partially imposed by the positions of multiple Fermi pockets, which itself can give rise to new order parameters, such as s+,− states or the state of the

Spin relaxation and antisymmetric exchange in n -doped III-V semiconductors

d_{x^2 - y^2 }

Breakup of a Stoner model for the two-dimensional ferromagnetic quantum critical point

symmetry. Other pairing states may appear on small pockets for long-range interactions, but they are expected to be sensitive to defects. We identify the competing antiferromagnetic order with the triplet exciton transition in the semimetallic background and discuss whether its coexistence with superconductivity explains the doping dependence of Tc.

Breakdown of the Fermi arcs in underdoped cuprates by incommensurate charge density waves

Experiment suggests that the divalent hexaborides are semiconductors and their ferromagnetism is due to a small concentration of intrinsic defects. The physics of these ferromagnets is put into perspective with recent observations implying ferromagnetic order in Mn-doped GaAs.

Gapless Fermi surfaces in superconducting Ce Co In 5

Recently, Kavokin [Phys. Rev. B 64, 075305 (2001)] suggested that the Dzyaloshinskii-Moriya interaction between localized electrons governs slow spin relaxation in n-doped GaAs in the regime close to the metal-insulator transition. We derive the correct spin Hamiltonian and apply it to the determination of spin dephasing time using the method of moments expansion. Our estimates would give longer than the observed values of the spin-relaxation time.

Competition between phonon superconductivity and Kondo screening in mixed valence and heavy fermion compounds

We analyze anew experiments on the NMR in cuprates and find an important information on their phase separation and its strip character hidden in the dependence of 1/63T1 on the degree of doping. In a broad class of materials, 1/63T1 is the sum of two terms: the temperature-independent one attributed to “ incommensurate” strips that occur at external doping and a “universal” temperature-dependent term ascribed to moving metallic and antiferromagnetic subphases. We argue that the frustrated first-order phase transition in a broad temperature interval bears a dynamical character.

Theory of Superconducting Alloys

Generalization of the results by A. V. Chubukov et al. [Phys. Rev. Lett. 90, 077002 (2003)] leads to the conclusion that the ferromagnetic quantum critical point cannot be described by a Stoner model because of a strong interplay between the paramagnetic fluctuations and the Cooper channel, at least in two dimensions.