In the early 1970s, Wilson developed the concept of a fully nonperturbative renormalization group transformation. When applied to the Kondo problem, this numerical renormalization group (NRG) method gave for the first time the full crossover from the high-temperature phase of a free spin to the low-temperature phase of a completely screened spin. The NRG method was later generalized to a variety of quantum impurity problems. The purpose of this review is to give a brief introduction to the NRG method, including some guidelines for calculating physical quantities, and to survey the development of the NRG method and its various applications over the last 30 years. These applications include variants of the original Kondo problem such as the non-Fermi-liquid behavior in the two-channel Kondo model, dissipative quantum systems such as the spin-boson model, and lattice systems in the framework of the dynamical mean-field theory.

http://www.physics.udel.edu/~bnikolic/QTTG/NOTES/NRG/BULLA=numerical_renormalization_group_method_for_quantum_impurity_systems.pdf

Numerical renormalization group method for quantum impurity systems

Gauge fields in graphene

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

https://link.aps.org/accepted/10.1103/RevModPhys.89.025006

Interface-induced phenomena in magnetism

A review of new developments in theoretical and experimental electronic-structure investigations of half-metallic ferromagnets (HMFs) is presented. Being semiconductors for one spin projection and metals for another, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMFs by the peculiarities of their electronic structure and chemical bonding is discussed. The effects of electron-magnon interaction in HMFs and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Special attention is paid to the appearance of nonquasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-the-art electronic calculations for correlated d-systems are discussed, and results for specific HMFs (Heusler alloys, zinc-blende structure compounds, CrO2, and Fe3O4) are reviewed.

/pdf/half-metallic-ferromagnets-from-band-structure-to-many-body-1hrl8mp5lm.pdf

Half-metallic ferromagnets: From band structure to many-body effects

Based on density functional calculations, optimized structures of graphite oxide are found for various coverages by oxygen and hydroxyl groups, as well as their ratio corresponding to the minimum of total energy. The model proposed describes well-known experimental results. In particular, it explains why it is so difficult to reduce the graphite oxide up to pure graphene. Evolution of the electronic structure of graphite oxide with the coverage change is investigated.

Modeling of Graphite Oxide

The pinning of the Fermi level to the Van Hove singularity and the formation of flat bands in the two-dimensional t - t' Hubbard model is investigated by the renormalization group technique. The "Van Hove" scenario of non-Fermi-liquid behavior for high-T c compounds can take place in a broad enough range of the hole concentrations. The results are in qualitative agreement with the recent angle-resolved photoemission spectroscopy data on La 2 CuO 4 .

http://arxiv.org/pdf/cond-mat/0110516.pdf

Robustness of the Van Hove scenario for high-T(c) superconductors.

Half-Metallic Ferromagnets: From Band Structure to Many-Body Effects

Peculiarities of transport properties of three- and two-dimensional half-metallic ferromagnets are investigated, which are connected with the absence of spin-flip scattering processes. The temperature and magnetic field dependences of resistivity in various regimes are calculated. The resistivity is proportional to T
9/2 for T < T
* and to T
7/2 for T > T
*, T* being the crossover temperature for longitudinal scattering processes. The latter scale plays also an important role in magnetoresistance. The contribution of non-quasiparticle (incoherent) states to the transport properties is discussed. It is shown that they can dominate in the temperature dependence of the impurity-induced resistivity and in the tunnel junction conductivity.

Temperature dependences of resistivity and magnetoresistivity for half-metallic ferromagnets

Formation of the Kondo state in the general two-band Anderson model has been investigated within the numerical renormalization group calculations. The Abrikosov-Suhl resonance is essentially asymmetric for the model with one electron per impurity (quarter filling case) in contrast with the one-band case. An external magnetic (pseudomagnetic) field breaking spin (orbital) degeneracy leads to asymmetric splitting and essential broadening of the many-body resonance. Unlike the standard Anderson model, the "spin-up" Kondo peak is pinned against the Fermi level, but not suppressed by the magnetic field.

https://arxiv.org/pdf/cond-mat/0411323.pdf

V. Yu. Irkhin

Papers

Half-metallic ferromagnets: From band structure to many-body effects

Robustness of the Van Hove scenario for high-T(c) superconductors.

Half-Metallic Ferromagnets: From Band Structure to Many-Body Effects

Temperature dependences of resistivity and magnetoresistivity for half-metallic ferromagnets

Kondo Resonance for Orbitally Degenerate Systems