On the Localization of Persistent Currents Due to Trapped Magnetic Flux at the Stacking Faults of Graphite at Room Temperature

Abstract

Granular superconductivity at high temperatures in graphite can emerge at certain two-dimensional (2D) stacking faults (SFs) between regions with twisted (around the c-axis) or untwisted crystalline regions with Bernal (ABA…) and/or rhombohedral (ABCABCA…) stacking order. One way to observe experimentally such 2D superconductivity is to measure the frozen magnetic flux produced by a permanent current loop that remains after removing an external magnetic field applied normal to the SFs. Magnetic force microscopy was used to localize and characterize such a permanent current path found in one natural graphite sample out of ∼50 measured graphite samples of different origins. The position of the current path drifts with time and roughly follows a logarithmic time dependence similar to the one for flux creep in type II superconductors. We demonstrate that a ≃10 nm deep scratch on the sample surface at the position of the current path causes a change in its location. A further scratch was enough to irreversibly destroy the remanent state of the sample at room temperature. Our studies clarify some of the reasons for the difficulties of finding a trapped flux in a remanent state at room temperature in graphite samples with SFs.

Peculiar Physics of Heavy-Fermion Metals: Theory versus Experiment

“…This is the case regardless of the diverse microscopic details characterizing these substances, such as crystal symmetry and structure defects. The explanation of this finding rests on the fact that the fermion condensation most readily occurs in substances hosting flat bands, see, e.g., [1,[5][6][7][8][9][10][11][12]. Experimental manifestations of the fermion condensation phenomena are varied, implying that different experimental techniques are most suitable for detecting and analyzing them.…”

“…Shown that the transition temperature T c is proportional to the Fermi velocity V F , V F ∝ T c , rather than N s (0) ∝ 1/V F ∝ T c ; Demonstrated that flat bands make T c ∝ g, with g being the coupling constant. It is of crucial importance to note that the flat band superconductivity has already been observed in twisted bilayer graphene, where due to the flat band, the transition temperature T c highly exceeds the limit dictated by the conventional BCS theory [5][6][7][8][9][10][11][12]. Thus, the basic task now is to attract more experimental groups to search for the room-T c superconductivity in graphite and other perspective materials.…”

“…Based on Equation (12) and Figures 3 and 4, we can construct the schematic T − B phase diagram of HF metals [52], reported in Figure 8. We assume here that at T = 0 and B = B c0 the system is approximately located at the FCQPT.…”

“…Strongly correlated Fermi systems such as heavy-fermion metals, graphene, and high-T c superconductors exhibit the non-Fermi-liquid (NFL) behavior. Theoretical predictions [1][2][3][4] and experimental data collected on many of these systems show that at low temperatures a portion of their excitation spectrum becomes approximately dispersionless, giving rise to so-called flat bands and high-T c superconductivity, see, e.g., [1,[5][6][7][8][9][10][11][12]. The emergence of flat bands at low T indicates that the system is close to a special quantum critical point, namely a topological fermion condensation quantum phase transition (FCQPT), leading to the formation of flat bands dubbed the fermion condensation (FC).…”

“…This fact, see Equation (2), leads to creating high-T c superconductors [1,[5][6][7][8][9][10][11][12]. This observation is supported by special features of high-T c superconductivity based on flat bands, namely that T c is proportional to the Fermi velocity V F ∝ 1/N s (0) V F ∝ T c , rather than N s (0) ∝ 1/V F ∝ T c as stated in standard BCS-like theories [13,16] as predicted [17].…”