00001 Coffee Break and Poster Setup — Friday , October 6 , 2006 10 : 30 AM-11 : 54 AM — Session SPS 1 Society of Physics Students (original) (raw)

Aspects of strong electron–phonon coupling in superconductivity of compressed metal hydrides MH6 (M = Mg, Ca, Sc, Y) with Im-3m structure

Journal of Applied Physics, 2021

Recently, YH 6 was synthesized as a first compound from theoretically predicted stable compressed MH 6 hydrides with bcc Im-3m crystal structures. Superconductivity of pressurized YH 6 was confirmed with critical temperature (T c) that is considerably lower than the predicted value by Migdal-Eliashberg (ME) theory. Here, we present theoretical reinvestigation of the superconductivity for selected MH 6 hydrides. Our results confirm that YH 6 and ScH 6 with Im-3m structure at corresponding GPa pressures are superconductors but with an anti-adiabatic character of superconducting ground state and a multiple-gap structure in one-particle spectrum. Transition into superconducting state is driven by strong electron-phonon coupling with phonons of H atom vibrations. Based on anti-adiabatic theory, calculated critical temperature T c in YH 6 is ≈ 231 K, i.e. just by ≈7 K higher than the experimental value. For ScH 6 the calculated critical temperature is T c ≈ 196 K. This value is by 27 K higher than a former theoretical prediction. Unexpected results concern CaH 6 and MgH 6 in Im-3m structure at corresponding GPa pressures. Calculated band structures (BS) indicate that in CaH 6 and MgH 6 the couplings to H stretching vibrations do not induce transitions into superconducting anti-adiabatic state and these hydrides remain stable in adiabatic metal-like state, which contradicts to former predictions of ME theory. These discrepancies are discussed in association with BS structure and a possible role of dorbitals on the involved metals, while we stress that the anti-adiabatic theory uses BS topology and its stability as a key input.

A QUALITATIVE OVERVIEW OF THE MECHANISMS OF SUPERCONDUCTIVITY

The mechanism of superconductivity continues to be one of the most fascinating and challenging topics in condense matter physics. The discovery of high-Tc cuprate superconductors and iron based superconductors has challenged the classical theories of condensed matter physics and opened a new chapter of strongly correlated electron systems. The key question of superconductivity is the nature of mechanism of pairing of carriers. The electron phonon interaction or spin fluctuations are considered to be central to the mechanism of superconductivity. In this article attempt has been made to highlights the brief outcome of various models and theories on the mechanism of superconductivity. I.Introduction Dutch scientist Heike Kammerlingh Onnes [1] discovered that electrical resistance of various metals e.g mercury, lead, tin and many others disappeared when the temperature was lowered below some critical value Tc. Meissner and Oschenfeld [2] observed that when a material is cooled in the presence of a magnetic field, on reaching its superconducting transition temperature (Tc) the magnetic flux is suddenly completely expelled from its interior. It means it exhibits perfect diamagnetism. Gorter [3, 4] put forward the idea of a two fluid model, in which the electron gas within the superconductor has two components. One component has no entropy and carries the supercurrent while the other component has all the entropy and behaves like a normal electron gas. Below the super conducting transition temperature, the superconducting electrons short out the normal electrons so that the electrical resistance is zero. These two features were captured in the equation proposed by London brothers [5], who first realized the quantum character of the phenomenon. Ginzburg and Landau [6] created a theory describing the transition between the superconducting and normal phases. Although the Ginzburg and Landau theory explained the macroscopic properties of superconductors, the microscopic properties remain unsolved. Bardeen, Cooper and Schrieffer created microscopic theory (BCS theory) [7] which describe conventional superconductors in the low temperature and low magnetic field regime. According to BCS theory, the superconductors at below Tc have an energy gap equal to binding energy of the Cooper pair, which dominates the transition temperature. The binding energy of the Cooper pair depends on the density of electron states at the Fermi surface, and on the strength of electron phonon interaction. High temperature superconductors are characterized by a layered two dimensional superconducting condensate and unique features that are very different from conventional superconducting materials. Recent studies [8, 9] reveal that the theoretical explanation for copper and iron superconductors could be the same and could even apply to other materials. The spin fluctuation mechanism of high-Tc superconductivity in copper oxide compound is determined by the high intensity of the antiferromagnetic exchange interaction. According to spin fluctuation mechanism [10], the pairing wave function of cuprate high-Tc superconductor should have d-wave symmetry. But unfortunately, some reports supported the d-symmetry for the high-Tc superconductors whereas others supported the s-symmetry. The survey of the mechanism of superconductivity [11] emphasized that all models used the conception of pairing with the subsequent formation of Bose-Condensate at Tc irrespective of the nature of the resulting attraction.

A QUALITATIVE OVERVIEW OF THE MECHANISMS OF SUPERCONDUCTIVITY SHAILAJ KUMAR SHRIVASTAVA

The mechanism of superconductivity continues to be one of the most fascinating and challenging topics in condense matter physics. The discovery of high-Tc cuprate superconductors and iron based superconductors has challenged the classical theories of condensed matter physics and opened a new chapter of strongly correlated electron systems. The key question of superconductivity is the nature of mechanism of pairing of carriers. The electron phonon interaction or spin fluctuations are considered to be central to the mechanism of superconductivity. In this article attempt has been made to highlights the brief outcome of various models and theories on the mechanism of superconductivity. I.Introduction Dutch scientist Heike Kammerlingh Onnes [1] discovered that electrical resistance of various metals e.g mercury, lead, tin and many others disappeared when the temperature was lowered below some critical value Tc. Meissner and Oschenfeld [2] observed that when a material is cooled in the presence of a magnetic field, on reaching its superconducting transition temperature (Tc) the magnetic flux is suddenly completely expelled from its interior. It means it exhibits perfect diamagnetism. Gorter [3, 4] put forward the idea of a two fluid model, in which the electron gas within the superconductor has two components. One component has no entropy and carries the supercurrent while the other component has all the entropy and behaves like a normal electron gas. Below the super conducting transition temperature, the superconducting electrons short out the normal electrons so that the electrical resistance is zero. These two features were captured in the equation proposed by London brothers [5], who first realized the quantum character of the phenomenon. Ginzburg and Landau [6] created a theory describing the transition between the superconducting and normal phases. Although the Ginzburg and Landau theory explained the macroscopic properties of superconductors, the microscopic properties remain unsolved. Bardeen, Cooper and Schrieffer created microscopic theory (BCS theory) [7] which describe conventional superconductors in the low temperature and low magnetic field regime. According to BCS theory, the superconductors at below Tc have an energy gap equal to binding energy of the Cooper pair, which dominates the transition temperature. The binding energy of the Cooper pair depends on the density of electron states at the Fermi surface, and on the strength of electron phonon interaction. High temperature superconductors are characterized by a layered two dimensional superconducting condensate and unique features that are very different from conventional superconducting materials. Recent studies [8, 9] reveal that the theoretical explanation for copper and iron superconductors could be the same and could even apply to other materials. The spin fluctuation mechanism of high-Tc superconductivity in copper oxide compound is determined by the high intensity of the antiferromagnetic exchange interaction. According to spin fluctuation mechanism [10], the pairing wave function of cuprate high-Tc superconductor should have d-wave symmetry. But unfortunately, some reports supported the d-symmetry for the high-Tc superconductors whereas others supported the s-symmetry. The survey of the mechanism of superconductivity [11] emphasized that all models used the conception of pairing with the subsequent formation of Bose-Condensate at Tc irrespective of the nature of the resulting attraction.

Aspects of strong electron-phonon coupling in superconductivity of compressed metal hydrides MH6 with Im-3m structure

arXiv (Cornell University), 2021

Recently, YH 6 was synthesized as a first compound from theoretically predicted stable compressed MH 6 hydrides with bcc Im-3m crystal structures. Superconductivity of pressurized YH 6 was confirmed with critical temperature (T c) that is considerably lower than the predicted value by Migdal-Eliashberg (ME) theory. Here, we present theoretical reinvestigation of the superconductivity for selected MH 6 hydrides. Our results confirm that YH 6 and ScH 6 with Im-3m structure at corresponding GPa pressures are superconductors but with an anti-adiabatic character of superconducting ground state and a multiple-gap structure in one-particle spectrum. Transition into superconducting state is driven by strong electron-phonon coupling with phonons of H atom vibrations. Based on anti-adiabatic theory, calculated critical temperature T c in YH 6 is ≈ 231 K, i.e. just by ≈7 K higher than the experimental value. For ScH 6 the calculated critical temperature is T c ≈ 196 K. This value is by 27 K higher than a former theoretical prediction. Unexpected results concern CaH 6 and MgH 6 in Im-3m structure at corresponding GPa pressures. Calculated band structures (BS) indicate that in CaH 6 and MgH 6 the couplings to H stretching vibrations do not induce transitions into superconducting anti-adiabatic state and these hydrides remain stable in adiabatic metal-like state, which contradicts to former predictions of ME theory. These discrepancies are discussed in association with BS structure and a possible role of dorbitals on the involved metals, while we stress that the anti-adiabatic theory uses BS topology and its stability as a key input.

Basic Foundations of the Microscopic Theory of Superconductivity

Arxiv preprint cond-mat/0603784, 2006

A new approach based on macro-orbital representation of a conduction electron in a solid has been used to discover some untouched aspects of the phonon induced attraction between two electrons and to lay the basic foundations of a general theory of superconductivity applicable to widely different solids. To this effect we first analyze the net hamiltonian, H(N), of N conduction electrons to identify its universal part, H o (N) (-independent of the nature of a specific solid or a specific class of solids), and then study the states of H o (N) to conclude that superconductivity originates, basically, from an interplay between the zero-point force (f o) of conduction electrons in their ground state and the inter-atomic forces (f a) which decide the lattice structure. This renders a kind of mechanical strain in the lattice which serves as the main source of phonon induced inter-electron attraction responsible for the formation of Cooper type pairs and the onset of superconductivity below certain temperature T c. We determine the binding energy of such pairs and find a relation for T c which not only accounts for the highest experimental T c ≈ 135K that we know today but also indicates that superconductivity may, in principle, occur at room temperature. It is evident that electrical strain in the lattice (i.e. electrical polarization of the lattice constituents produced by the charge of conducting electrons) can have an added contribution to the phonon induced attraction of two electrons. Our theoretical framework not only incorporates BCS model but also provides microscopic basis for the two well known phenomenologies of superconductivity, viz., the two fluid theory and Ψ−theory. In addition, it also corroborates a recent idea that superconducting transition is basically a quantum phase transition.

Superconductivity 2022

Metals

Superconductivity in metals and alloys, i.e., conventional superconductivity, has seen many new developments in recent years, leading to a renewed interest in the principles of superconductivity and the search for new materials. The most striking discoveries include the near-room-temperature superconductivity in metal hydrides (LaH10) under pressure, the extreme stability of superconductivity in NbTi up to 261 GPa pressure, the discovery of high-entropy alloy (HEA) superconductor materials, and the machine learning prediction of new superconducting materials. Other interesting research concerns the properties of 2D superconductors, topological superconductors, e.g., in hybrid systems, and the use of nanotechnology to create nanowires and nanostructures with new properties. Furthermore, and most importantly, the drive from new accelerator and fusion reactors for stronger superconducting magnets has lead to improved cable materials, showing the highest critical current densities ever....

Competition Between the Pseudogap and Superconducting States of Bi2Sr2Ca0.92Y0.08Cu...

Physical Review Letters, 2013

A pairing gap and coherence are the two hallmarks of superconductivity. In a classical BCS superconductor they are established simultaneously at T c . In the cuprates, however, an energy gap (pseudogap) extends above T c [1, 2, 3,. The origin of this gap is one of the central issues in high temperature superconductivity. Recent experimental evidence demonstrates that the pseudogap and the superconducting gap are associated with different energy scales . It is however not clear whether they coexist independently or compete . In order to understand the physics of cuprates and improve their superconducting properties it is vital to determine whether the pseudogap is friend or foe of high temperature supercondctivity . Here we report evidence from angle resolved photoemission spectroscopy (ARPES) that the pseudogap and high temperature superconductivity represent two competing orders. We find that there is a direct correlation between a loss in the low energy spectral weight due to the pseudogap and a decrease of the coherent fraction of paired electrons. Therefore, the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing in the region of momentum space where the superconducting gap is largest. This leads to a very unusual state in the underdoped cuprates, where only part of the Fermi surface develops coherence.