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We consider the formation of polarons in three different situations: (1) the formation of a polaron in the crystal due to the electron-phonon interaction, (2) the formation of a polaron in the presence of a quiescent Fermi sea, and the (3) the formation of polarons in an interacting Fermi gas, and Fermi liquids, including charge and spin polarization or magnetic polarization. We develop the perturbation diagram approach to the polaron problem and show that the series of maximally crossed and repeated one-phonon (Tamm–Dancoff approximation) diagrams leads to the series of the ladder diagrams which is equivalent to the well-known Bethe–Salpeter equation. We show that the polaron mass cannot exceed the triple mass of an electron and the bound state for the electron-optical phonon interaction exists for any strength of the electron-phonon coupling constant in 3D, and the bound state always exists in 2D case even for the electron-acoustic phonon interaction. We consider the polaron effect in the presence of a quiescent Femi sea and show that the polaron effect leads both to the appearance of the insulating gap and the formation of the quasiparticles with the dispersion law similar to the quasiparticle spectrum in the BCS model of superconductivity. We show that the polarization effect in the case of interacting Fermi gas leads to the polaron caused by the exchange-correlation hole and the electron spectrum again is an insulating with the dispersion law of quasiparticles similar to the BCS model. We show that the interaction potential in polaronic effect in the case of Femi liquid is the same as the Landau interaction function for Fermi liquid. Moreover, we also consider spin and magnetic polaronic effects in Fermi liquids. We show that in the case of repulsive Hubbard model in the spin channel appears the spin gap with the spectrum of quasiparticles again similar to the BCS model of quasiparticles.

We present the preparation of MnGe coated Si nanowires (NWs) using molecular beam epitaxy. Uniformly dispersed silicon NW surfaces were initially covered by Ge, and then Si/MnGe core/shell NW heterostructures were fabricated by solid phase epitaxial growth. Morphology and the magnetic properties of the Si/MnGe core/shell NWs were investigated. The results of scanning electron microscope and transmission electron microscope revealed that the shell layer of NWs was agglomerated to form clusters, which were mainly comprised of Mn5Ge3 phase. Vibrating magnetometer and X-band ferromagnetic resonance measurements indicate that the Si/MnGe core/shell NWs exhibited a slight shape anisotropy along the geometrical wire axis.

The current-voltage characteristics of Bi2Sr2Ca2Cu3O10/Ag multifilamentary tapes were measured at different temperatures close to the critical temperature and in different applied magnetic fields up to 1000 Oe. The data were interpreted in terms of thermally activated flux creep by using an intermediate phenomenological model that takes into account the collective pinning. Temperature and field dependences of the pinning potential and of the collective pinning exponent were determined through numerical analysis.

In the BCS framework, exact expressions for the ratio between the jump in the specific heat at T c and the normal phase specific heat are derived within the Van Hove singularity scenario. Analytical results are obtained for an isotropic s-wave and anisotropic d-wave pairing symmetries. Graphical solutions of the ratio as functions of ω D /T c and E F /T c , where ω D is the cutoff energy and E F is the Fermi energy, show significant deviations from the BCS value of 1.43.

Migration process of spin polarization over disordered system of impurity nuclei is considered. The long time asymptotics of the process is investigated on the basis of a new numerical simulation method, which includes periodic continuation of the system without periodic continuation of the initial condition. It is shown that the long time asymptotics has a diffusive character. The concentration dependence of the diffusion tensor is studied.

The structural, magnetic, elastic, mechanical, and thermodynamic properties of BaC and BaN compounds in different phases were studied using first-principle calculations based on spin-polarized density functional theory within the generalized gradient approximation (GGA-PBEsol) and the modified Becke–Johnson approach (mBJ-GGA-PBEsol) for the exchange-correlation energy and potential. The following phases—rock-salt (NaCl), CsCl, zinc blende (ZB), NiAs- and WZ-type hexagonal, tetragonal (P4/nmm), and orthorombic (Pnma) phases of BaC and BaN compounds—were considered. We obtained that Pnma phase has the lowest energy configuration as a function of the volume for both the BaN and BaC compounds. The ferromagnetic phase is energetically favored with respect to the non-magnetic phase in the BaN and BaC compounds, except for the CsCl phase in the BaC compound. Considering the phonon dynamics of BaN and BaC compounds in the Pnma, NaCl, ZB, and WZ phases, we observed that the BaN and BaC compounds in the Pnma, NaCl, and ZB phases are dynamically stable. The calculated elastic properties for the Pnma, NaCl, and ZB phases show that they are elastically stable. The Pnma phase for the BaN and BaC compounds, which is a new phase was found to be dynamically and elastically stable. The BaN and BaC compounds exhibit half-metallic behavior in the Pnma, NaCl, and ZB phases. The half-metallic and magnetic character found in the BaN and BaC compounds are attributed to the presence of spin-polarized 2p orbitals of the nitrogen and carbon atoms, respectively. We found that BaN and BaC compounds are half-metallic ferromagnets with magnetic moment of 1 μB and 2 μB per formula unit, respectively. Using the GGA-PBEsol (mBJ-GGA-PBEsol) approach, our calculated half-metallic gaps for BaN and BaC compounds are 0.22 eV (0.54 eV) and 0.32 eV (0.44 eV) in the Pnma phase, 0.23 eV (1.32 eV) and 0.35 eV (1.00 eV) in the NaCl phase, and 0.38 eV (1.54 eV) and 0.50 eV (1.57 eV) in the ZB phase, respectively.

The possibility of a spontaneous spin-triplet paired phase of the Fulde-Ferrell-Larkin-Ovchinnikov type is studied. As it is shown in a system with the dominant interband pairing and two distinct Fermi surface sheets, the Fermi wave-vector mismatch can be compensated by a nonzero center-of-mass momentum of the Cooper pairs. This idea is examined with the use of a model which describes the two hole-like bands in the iron-based superconductor. It is shown that for the proper range of model parameters, the minima of the free energy appear which correspond to a nonzero Cooper pair momentum. Different superconducting gap symmetries are analyzed, and the corresponding phase diagrams are shown.

Synthesis, structure, and magnetic properties of copper bromide CuL2Br2 with 3-amino-4-ethoxycarbonylpyrazole (L) are presented. The CuL2Br2 is a 1D polymer complex and crystal structure has triclinic symmetry with space group P-1. The examined compound has thermochromic properties (russet ↔ light red), and its magnetic properties are changed from paramagnetic to nonresonance and ferromagnetic ones after heat treatment (300 → 77 → 300 K) or pressure treatment (1 → 3,000 → 1 atm).

The magnetoresistance due to the disorder in the inhomogeneous systems can be studied using the classical model of resistor networks. In this work, the magnetoresistance of an inhomogeneous lattice is simulated using this model. The inhomogeneous network consists of two materials with different resistivity. Maxwell’s equations have been solved for a four-terminal resistor unit first, and then the system magnetoresistance is calculated based on the resistor network model. Results show that the magnetoresistance diverges for some configurations at a specific magnetic field that is promising for sensors applications. It is also exhibited that the magnetoresistance depends on the specific resistance ratios as well as the amount and alignment of the doped material.

Cơ chế chuyển tiếp trong các siêu dẫn cuprate nhiệt độ cao là một câu đố nổi bật. Một đề xuất trước đó về vai trò của các chế độ không tuyến tính trong sự không ổn định của mạng lưới đối với cơ chế kết đôi vi mô trong các siêu dẫn cuprate nhiệt độ cao (Lee, J. Supercond. Nov. Magn. 23(3), 333; 2009) được xem xét lại để cung cấp một cơ chế khả thi cho siêu dẫn trong các cuprate này thông qua một dao động mạng lưới không thường thấy, trong đó một electron chủ yếu tương tác với một chế độ Q2 phi tuyến của các cụm oxy trong các mặt phẳng CuO2. Đã chỉ ra rằng tương tác này có đối xứng d-wave rõ ràng và dẫn đến một sự liên kết gián tiếp của đối xứng d-wave giữa các electron. Là một phần tiếp theo của Lee (J. Supercond. Nov. Magn. 23(3), 333; 2009), trong bài báo này, chúng tôi báo cáo sự suy diễn chi tiết của phương trình lỗ hổng siêu dẫn và các giải pháp số cho nhiệt độ chuyển tiếp, vốn được tích hợp nội tại vào mô hình Hubbard mở rộng (EHM) được gọi là. Một đặc điểm độc đáo trong EHM là nhiệt độ chuyển tiếp có sự phụ thuộc nội tại vào k. Ngoài ra, các giải pháp cho lỗ hổng siêu dẫn được hạn chế trong các khu vực cụ thể trong vùng Brillouin đầu tiên (1BZ). Thật dễ dàng để kỳ vọng rằng EHM tự nhiên thừa hưởng một không gian tham số khổng lồ, nơi mà các kết quả được đo thực nghiệm, chẳng hạn như mái vòm siêu dẫn nổi tiếng và sơ đồ pha từ tán xạ Raman điện tử (Sacuto et al., Rep. Prog. Phys. 76(2), 022502; 2013) có thể được tiếp nhận. Mô hình EHM do đó cung cấp một kênh khả thi để tìm kiếm hoặc xác nhận bất kỳ dấu hiệu nào trong các phép đo thực nghiệm nhạy cảm với điểm k.