Iron based superconductors: magnetism, superconductivity and electronic structure

Angle resolved photoemission spectroscopy (ARPES) reveals the features of the electronic structure of quasi-two-dimensional crystals, which are crucial for the formation of spin and charge ordering and determine the mechanisms of electron-electron interaction, including the superconducting pairing. The newly discovered iron based superconductors (FeSC) promise interesting physics that stems, on one hand, from a coexistence of superconductivity and magnetism and, on the other hand, from complex multi-band electronic structure. In this review I want to give a simple introduction to the FeSC physics, and to advocate an opinion that all the complexity of FeSC properties is encapsulated in their electronic structure. For many compounds, this structure was determined in numerous ARPES experiments and agrees reasonably well with the results of band structure calculations. Nevertheless, the existing small differences may help to understand the mechanisms of the magnetic ordering and superconducting pairing in FeSC.Comment: Invited Revie

Hybridization and interference effects for localized superconducting states in strong magnetic field

Within the Ginzburg-Landau model we study the critical field and temperature enhancement for crossing superconducting channels formed either along the sample edges or domain walls in thin-film magnetically coupled superconducting - ferromagnetic bilayers. The corresponding Cooper pair wave function can be viewed as a hybridization of two order parameter (OP) modes propagating along the boundaries and/or domain walls. Different momenta of hybridized OP modes result in the formation of vortex chains outgoing from the crossing point of these channels. Near this crossing point the wave functions of the modes merge giving rise to the increase in the critical temperature for a localized superconducting state. The origin of this critical temperature enhancement caused by the wave function squeezing is illustrated for a limiting case of approaching parallel boundaries and/or domain walls. Using both the variational method and numerical simulations we have studied the critical temperature dependence and OP structure vs the applied magnetic field and the angle between the crossing channels.Comment: 12 pages, 13 figure

Field-asymmetric transverse magnetoresistance in a nonmagnetic quantum-size structure

A new phenomenon is observed experimentally in a heavily doped asymmetric quantum-size structure in a magnetic field parallel to the quantum-well layers - a transverse magnetoresistance which is asymmetric in the field (there can even be a change in sign) and is observed in the case that the structure has a built-in lateral electric field. A model of the effect is proposed. The observed asymmetry of the magnetoresistance is attributed to an additional current contribution that arises under nonequilibrium conditions and that is linear in the gradient of the electrochemical potential and proportional to the parameter characterizing the asymmetry of the spectrum with respect to the quasimomentum.Comment: 10 pages, 5 figures. For correspondence, mail to [email protected]

Excitonic BCS-BEC crossover at finite temperature: Effects of repulsion and electron-hole mass difference

The BCS to Bose-Einstein condensation (BEC) crossover of electron-hole (e-h) pairs in optically excited semiconductors is studied using the two-band Hubbard model with both repulsive and attractive interactions. Applying the self-consistent t-matrix approximation combined with a local approximation, we examine the properties of a normal phase and an excitonic instability. The transition temperature from the normal phase to an e-h pair condensed one is studied to clarify the crossover from an e-h BCS-like state to an excitonic Bose-Einstein condensation, which takes place on increasing the e-h attraction strength. To investigate effects of the repulsive interaction and the e-h mass difference, we calculate the transition temperature for various parameters of the interaction strengths, the e-h particle density, and the mass difference. While the transition temperature in the e-h BCS regime is sufficiently suppressed by the repulsive interaction, that of the excitonic BEC is largely insensitive to it. We also show quantitatively that in the whole regime the mass difference leads to large suppression of the transition temperature.Comment: 8 pages, 7 figures, to be published in Phys. Rev.

Решение задачи о наклонной производной для уравнения Лаврентьева – Бицадзе в полуплоскости

The paper solves the boundary value problem of an oblique derivative for the Lavrent'ev – Bitsadze equation in a half-plane. The Lavrent'ev – Bitsadze equation is an equation of mixed (elliptic-hyperbolic) type. Mixed-type equations arise when solving many applied problems (for example, when simulating transonic flows of a compressible medium).In the paper, the domain of ellipticity is a half-plane, and that of hyperbolicity is its adjacent strip. On one of the straight lines bounding the strip, an oblique derivative is specified (in the direction that forms an acute angle with this straight line), and on the other straight line, which is the interface between the strip and the half-plane, the solutions are matched by boundary conditions of the fourth kind. In the hyperbolicity strip, the solution is represented by the d'Alembert formula, and in the half-plane, where the equation is elliptic, the bounded solution is represented by the Poisson integral with unknown density. For this unknown density of the Poisson integral, a singular integral equation is obtained, which is reduced to the Riemann boundary value problem with a shift for holomorphic functions. The solution of the Riemann problem is reduced to the solution of two functional equations. Solutions of these functional equations and the Sokhotsky formula for an integral of Cauchy type allowed us to find the unknown density of the Poisson integral. This allowed us to find a solution to the oblique derivative problem as the sum of a functional series (up to an arbitrary constant term).В работе решается краевая задача о наклонной производной для уравнения Лаврентьева – Бицадзе в полуплоскости. Уравнение Лаврентьева – Бицадзе является уравнением смешанного (эллиптико – гиперболического) типа. Уравнения смешанного типа возникают при решении многих задач прикладного характера (например, при моделировании околозвуковых течений сжимаемой среды).В работе областью эллиптичности является полуплоскость, а областью гиперболичности – примыкающая к ней полоса. На одной из прямых, ограничивающих полосу, задана наклонная производная (в направлении, образующим острый угол с этой прямой), а на другой прямой – границе раздела полосы и полуплоскости – решения сопрягаются краевыми условиями четвертого рода. В полосе гиперболичности решение представлено формулой Даламбера, а в полуплоскости, где уравнение является эллиптическим, ограниченное решение представлено интегралом Пуассона с неизвестной плотностью. Для этой неизвестной плотности интеграла Пуассона получено сингулярное интегральное уравнение, которое сведено к краевой задаче Римана со сдвигом для голоморфных функций. Решение задачи Римана сведено к решению двух функциональных уравнений. Решения этих функциональных уравнений и формулы Сохоцкого для интеграла типа Коши позволили найти неизвестную плотность интеграла Пуассона. А это позволило найти решение задачи о наклонной производной в виде суммы функционального ряда (с точностью до произвольного постоянного слагаемого)

Biordered superconductivity and strong pseudogap state

Interrelation between the two-particle and mean-field problems is used to describe the strong pseudogap and superconducting states in cuprates. We present strong pseudogap state as off-diagonal short-range order (ODSRO) originating from quasi-stationary states of the pair of repulsing particles with large total momentum (K - pair). Phase transition from the ODSRO state into the off-diagonal long-range ordered (ODLRO) superconducting state is associated with Bose-Einstein condensation of the K - pairs. A checkerboard spatial order observable in the superconducting state in the cuprates is explained by a rise of the K - pair density wave. A competition between the ODSRO and ODLRO states leads to the phase diagram typical of the cuprates. Biordered superconducting state of coexisting condensates of Cooper pairs with zero momentum and K - pairs explains some properties of the cuprates observed below Tc: Drude optical conductivity, unconventional isotope effect and two-gap quasiparticle spectrum with essentially different energy scales.Comment: 11 pages, 4 fugure

Pseudogap from ARPES experiment: three gaps in cuprates and topological superconductivity

A term first coined by Mott back in 1968 a `pseudogap' is the depletion of the electronic density of states at the Fermi level, and pseudogaps have been observed in many systems. However, since the discovery of the high temperature superconductors (HTSC) in 1986, the central role attributed to the pseudogap in these systems has meant that by many researchers now associate the term pseudogap exclusively with the HTSC phenomenon. Recently, the problem has got a lot of new attention with the rediscovery of two distinct energy scales (`two-gap scenario') and charge density waves patterns in the cuprates. Despite many excellent reviews on the pseudogap phenomenon in HTSC, published from its very discovery up to now, the mechanism of the pseudogap and its relation to superconductivity are still open questions. The present review represents a contribution dealing with the pseudogap, focusing on results from angle resolved photoemission spectroscopy (ARPES) and ends up with the conclusion that the pseudogap in cuprates is a complex phenomenon which includes at least three different `intertwined' orders: spin and charge density waves and preformed pairs, which appears in different parts of the phase diagram. The density waves in cuprates are competing to superconductivity for the electronic states but, on the other hand, should drive the electronic structure to vicinity of Lifshitz transition, that could be a key similarity between the superconducting cuprates and iron based superconductors. One may also note that since the pseudogap in cuprates has multiple origins there is no need to recoin the term suggested by Mott.Comment: invited review, more info at http://www.imp.kiev.ua/~kor

Electronic Structure, Local Moments and Transport in Fe_2VAl

Local spin density approximation calculations are used to elucidate electronic and magnetic properties of Heusler structure Fe_2VAl. The compound is found to be a low carrier density semimetal. The Fermi surface has small hole pockets derived from a triply degenerate Fe derived state at Gamma compensated by an V derived electron pocket at the X point. The ideal compound is found to be stable against ferromagnetism. Fe impurities on V sites, however, behave as local moments. Because of the separation of the hole and electron pockets the RKKY interaction between such local moments should be rapidly oscillating on the scale of its decay, leading to the likelihood of spin-glass behavior for moderate concentrations of Fe on V sites. These features are discussed in relation to experimental observations of an unusual insulating state in this compound.Comment: 16 pages, RevTeX, 5 figure