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Superlight small bipolarons in the presence of strong Coulomb repulsion  [PDF]
J. P. Hague,P. E. Kornilovitch,J. H. Samson,A. S. Alexandrov
Physics , 2006, DOI: 10.1103/PhysRevLett.98.037002
Abstract: We study a lattice bipolaron on a staggered triangular ladder and triangular and hexagonal lattices with both long-range electron-phonon interaction and strong Coulomb repulsion using a novel continuous-time quantum Monte-Carlo (CTQMC) algorithm extended to the Coulomb-Frohlich model with two particles. The algorithm is preceded by an exact integration over phonon degrees of freedom, and as such is extremely efficient. The bipolaron effective mass and bipolaron radius are computed. Lattice bipolarons on such lattices have a novel crablike motion, and are small but very light in a wide range of parameters, which leads to a high Bose-Einstein condensation temperature. We discuss the relevance of our results with current experiments on cuprate high-temperature superconductors and propose a route to room temperature superconductivity.
Superlight small bipolarons  [PDF]
J. P. Hague,P. E. Kornilovitch,J. H. Samson,A. S. Alexandrov
Physics , 2007, DOI: 10.1088/0953-8984/19/25/255214
Abstract: Recent angle-resolved photoemission spectroscopy (ARPES) has identified that a finite-range Fr\"ohlich electron-phonon interaction (EPI) with c-axis polarized optical phonons is important in cuprate superconductors, in agreement with an earlier proposal by Alexandrov and Kornilovitch. The estimated unscreened EPI is so strong that it could easily transform doped holes into mobile lattice bipolarons in narrow-band Mott insulators such as cuprates. Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the total energy, effective mass, pair radius, number of phonons and isotope exponent of lattice bipolarons in the region of parameters where any approximation might fail taking into account the Coulomb repulsion and the finite-range EPI. The effects of modifying the interaction range and different lattice geometries are discussed with regards to analytical strong-coupling/non-adiabatic results. We demonstrate that bipolarons can be simultaneously small and light, provided suitable conditions on the electron-phonon and electron-electron interaction are satisfied. Such light small bipolarons are a necessary precursor to high-temperature Bose-Einstein condensation in solids. The light bipolaron mass is shown to be universal in systems made of triangular plaquettes, due to a novel crab-like motion. Another surprising result is that the triplet-singlet exchange energy is of the first order in the hopping integral and triplet bipolarons are heavier than singlets in certain lattice structures at variance with intuitive expectations. Finally, we identify a range of lattices where superlight small bipolarons may be formed, and give estimates for their masses in the anti-adiabatic approximation.
Route to Room-Temperature Superconductivity from a Practical Point of View  [PDF]
A. Mourachkine
Physics , 2007,
Abstract: To synthesize a new superconductor which has a critical temperature, Tc, exceeding the room temperature, one needs to know what chemical components to start with. This chapter presents analysis of experimental data which allow one to draw a conclusion about components and the structure of a potential room-temperature superconductor. The two essential components of a room temperature superconductor are large organic molecules (polymers, tissues) and atoms/molecules which are magnetic in the intercalated state. This conclusion is fully based on experimental facts known today, and does not require any assumptions about the mechanism of room-temperature superconductivity. This, however, does not mean that to synthesize a room-temperature superconductor is an easy task.
Flat band in topological matter: possible route to room-temperature superconductivity  [PDF]
G. E. Volovik
Physics , 2011, DOI: 10.1007/s10948-013-2221-5
Abstract: Topological media are systems whose properties are protected by topology and thus are robust to deformations of the system. In topological insulators and superconductors the bulk-surface and bulk-vortex correspondence gives rise to the gapless Weyl, Dirac or Majorana fermions on the surface of the system and inside vortex cores. In gapless topological media, the bulk-surface and bulk-vortex correspondence produce topologically protected gapless fermions without dispersion - the flat band. Fermion zero modes forming the flat band are localized on the surface of topological media with protected nodal lines and in the vortex core in systems with topologically protected Fermi points (Weyl points). Flat band has an extremely singular density of states, and this property may give rise in particular to surface superconductivity which in principle could exist even at room temperature.
Superlight bipolarons and a checkerboard d-wave condensate in cuprates  [PDF]
A. S. Alexandrov
Physics , 2003, DOI: 10.1023/B:JOSC.0000011840.24281.eb
Abstract: The seminal work by Bardeen, Cooper and Schrieffer taken further by Eliashberg to the intermediate coupling solved the problem of conventional superconductors about half a century ago. The Froehlich and Jahn-Teller electron-phonon interactions were identified as an essential piece of physics in all novel superconductors. The BCS theory provides a qualitatively correct description of some of them like magnesium diborade and doped fullerenes (if the polaron formation is taken into account). However, cuprates remain a problem. Here I show that the bipolaron extension of the BCS theory to the strong-coupling regime could be a solution. Low-energy physics in this regime is that of small 'superlight' bipolarons, which are real-space mobile bosonic pairs dressed by phonons. The symmetry and space modulations of the order parameter are explained in the framework of the bipolaron theory. A d-wave Bose-Einstein condensate of bipolarons reveals itself as a checkerboard modulation of the hole density and of the gap below Tc.
Superlight bipolarons and criterion of BCS-BEC crossover in cuprates  [PDF]
A. S. Alexandrov
Physics , 2003, DOI: 10.1142/S0217979203020910
Abstract: Most of the proposed models of high-temperature superconductivity (HTSC) are based on the short-range electron-electron correlations or/and on a short-range electron-phonon interaction. However, in the cuprates the screening is poor due to a low carrier density, layered crystal structure, and high ionicity of the lattice. We develop further the bipolaron model of HTSC, which explicitly takes into account the long-range origin of all interactions. The long-range electron-phonon (Froehlich) interaction binds carriers into real space pairs (small bipolarons) with surprisingly low mass but sufficient binding energy, while the long-range Coulomb repulsion keeps them from forming larger clusters. The model has explained many key features of cuprates. Here it is shown that real-space pairing takes place in many cuprates at variance with some (incorrect) criteria of the BCS-BEC crossover.
Indications of room-temperature superconductivity at a metal-PZT interface  [PDF]
Dhruba Dasgupta
Physics , 2010,
Abstract: We report the observation of an exceptionally large room-temperature electrical conductivity in silver and aluminum layers deposited on a lead zirconate titanate (PZT) substrate. The surface resistance of the silver-coated samples also shows a sharp change near 313 K. The results are strongly suggestive of a superconductive interfacial layer, and have been interpreted in the framework of Bose-Einstein condensation of bipolarons as the suggested mechanism for high-temperature superconductivity in cuprates.
Room-Temperature Superconductivity  [PDF]
A. Mourachkine
Physics , 2006,
Abstract: This is the first book on the subject of room-temperature superconductivity. The main purpose of the book is twofold. First, to show that, under suitable conditions, superconductivity can occur above room temperature. Secondly, to present general guidelines on how to synthesize a room temperature superconductor. The book begins with an introduction into the physics of the superconducting state and superconducting materials. The mechanisms of conventional, half-conventional and unconventional superconductivity are discussed in the following chapters. The last three chapters of the book are devoted to room temperature superconductivity. In Chapter 2, an attempt to review the basic properties of the superconducting state independently of any specific mechanism is made for the first time. In addition, four principles of superconductivity valid for any type of superconductivity are introduced in Chapter 4. The book is mainly addressed to specialists in materials science and in the field of superconductivity. At the same time, students will also benefit from reading the first nine chapters of the book.
Bloch waves of small high-Tc bipolarons  [PDF]
A. S. Alexandrov
Physics , 2002, DOI: 10.1002/pssb.200301762
Abstract: Over the last decade several competing models of high-temperature superconductivity were proposed, most of them with short-range interactions. We review a more realistic model with strong on-site repulsive correlations, the Coulomb and strong finite-range electron-phonon interactions. Bipolarons in the model exist in the itinerant Bloch states at temperatures below about half of the characteristic phonon frequency. Depending on the ratio of the inter-site Coulomb repulsion and the polaron level shift the ground state of the model is a polaronic Fermi (or Luttinger) liquid, bipolaronic high-Tc superconductor, or charge-segregated insulator for the strong, intermediate, and weak Coulomb repulsion, respectively. Two particular lattices are analysed in detail: a chain with the finite range electron-phonon interaction and a zig-zag ladder. Charge carriers in the ladder are superlight mobile intersite bipolarons. They propagate coherently without emission or absorption of phonons with about the same mass as single polarons. The model describes key features of the cuprates, in particular their Tc values, different isotope effects, normal state pseudogaps, and spectral functions measured in tunnelling and photoemission.
Strong-coupling theory of high-temperature superconductivity and colossal magnetoresistance  [PDF]
A. S. Alexandrov
Physics , 2005, DOI: 10.1117/12.614087
Abstract: We argue that the extension of the BCS theory to the strong-coupling regime describes the high-temperature superconductivity of cuprates and the colossal magnetoresistance (CMR) of ferromagnetic oxides if the phonon dressing of carriers and strong attractive correlations are taken into account. The long-range Froehlich electron-phonon interaction has been identified as the most essential in cuprates providing "superlight" lattice polarons and bipolarons. Here some kinetic, magnetic, and more recent thermomagnetic normal state measurements are interpreted in the framework of the strong-coupling theory, including the Nernst effect and normal state diamagnetism. Remarkably, a similar strong-coupling approach offers a simple explanation of CMR in ferromagnetic oxides. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair-breaking in the ferromagnetic phase account for the first-order ferromagnetic phase transition, CMR, isotope effects, and pseudogaps in doped manganites. Here we propose an explanation of the phase coexistence and describe the shape of resistivity of manganites near the transition in the framework of the strong-coupling approach.
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