arXiv subjects
Publications and source records attributed to N. Zen.
I propose the index $z$, defined as $z=\sum\limits_{i=1}^{n} \mathrm{IF}_{i}$, where $\mathrm{IF}_{i}$ is the impact factor of the journal in the publication year of the $i$-th retracted paper. Whereas the $h$-index is a forward-looking measure of scientific achievement, the $z$-index quantifies scientific debt. The quantity $h-z$ may provide a more balanced assessment of scientific value.
In Fig. 4c, under the section titled "Pinning and thermally activated motion of vortices" in arXiv:2206.14108 and Nat Phys. 19, 1293 (2023) [1], Minkov and co-workers presented the time dependence of the magnetic moment of sulfur hydride (H$_{3}$S) under high pressure and argued that they had observed magnetic flux creep at 165 K, 180 K and 185 K. Flux creep is a phenomenon observed under the assumption that the material under study can trap magnetic flux, and thus, Fig. 4c serves as evidence that H$_{3}$S traps magnetic flux and is a high-temperature superconductor. The claim remains unchanged even in the recently published Author Correction [2] to Ref. [1]. However, Ref. [2] discloses an experimental protocol they used to collect the time-dependent magnetic moment data. In this Commentary Paper, we point out that the protocol is not applicable to H$_{3}$S under high pressure and propose an alternative protocol. The correct protocol demonstrates that the claim in Refs. [1,2] -- that their time-dependent magnetic moment data serve as evidence of "pinning and thermally activated motion of vortices" -- is indeed invalid.
In Ref. [1], Minkov et al reported the time dependence of magnetic moment of hydride materials under high pressure in a diamond anvil cell. Here we point out that the straight lines interpolated with the measurement results give the misleading impression that thermal flux creep in superconductors decays linearly with time. To dispel this misconception, we correctly interpolated logarithmic decay curves. Then, it has become apparent that there is no reason to assume that the measured magnetic moment originates from flux trapping, i.e., there is no reason to believe that persistent currents are circulating in hydride materials under high pressure and that they are superconductors.
By making periodic thru-holes in a suspended film, the phonon system can be modified. Motivated by the BCS theory, the technique -- so-called phonon engineering -- was applied to a metallic niobium sheet. It was found that its electrical resistance dropped to zero at 175 K, and the zero-resistance state persisted up to 290 K in the subsequent warming process. Despite the initial motivation, neither these high transition temperatures nor the phase transition with thermal hysteresis can be accounted for by the BCS theory. Therefore, we abandon the BCS theory. Instead, it turns out that the metallic holey sheet is partly oxidized to form a niobium-oxygen square lattice, which has points of resemblance to a copper-oxygen plane, the fundamental component of cuprate high-$T_{c}$ superconductors. Therefore, the pairing mechanism underlying this study should be related to that of cuprate high-$T_{c}$ superconductors, which we may not yet understand. In addition to the electrical results of zero resistance, the holey sheet exhibited a decrease in magnetization upon cooling, i.e., the Meissner effect. Moreover, the remnant magnetization was clearly detected at 300 K, which can only be attributed to persistent currents flowing in a superconducting sample. Thus, this study meets the established criteria for a conclusive demonstration of true superconductivity. Finally, the superconducting transition with the unambiguous thermal hysteresis is discussed. According to Halperin, Lubensky, and Ma, or HLM for short, any superconducting transition must $always$ be first order with thermal hysteresis because of the intrinsic fluctuating magnetic field. The HLM theory is very compatible with the highly oriented system harboring two-dimensional superconductivity.
Recently, the room-temperature superconductor (RTSC) was discovered as a two-dimensional (2D) square lattice made of a metal wherein positive charges, i.e. holes, were heavily concentrated. The experimental result for the critical magnetic field $H_{c}$ was fully consistent with the view on the RTSC that its lattice unit -- a metal island -- is filled with the Slater's atoms. Each Slater's atom has the expanded diameter of 14.5 nm in order to have perfect diamagnetism with the magnitude corresponding to a single flux quantum $\phi_{0}$. Its expanded orbit is associated with the fine structure constant $\alpha \approx \frac{1}{137}$. In this paper, another important critical value -- the critical current $I_{c}$ -- is reported. It was found that the supercurrent has achieved the ultimate speed of matter, i.e., the speed of light, $c$. Beginning with a warm-up exercise for the Bohr's atom, how the Slater's atom is formed and why the $c$ appears are shown. These considerations lead to a simple view on the pairing mechanism of superconductivity, which also gives an ample indication of the most mysterious physical number $\alpha$. Finally, it is shown that the proposed pairing mechanism in terms of London's canonical momentum naturally generates the perfect diamagnetic $\phi_{0}$ of the Slater's atom, and the superconducting energy gap $\Delta$ is predicted.
By drilling periodic thru-holes in a suspended film, the phonon system can be modified. Being motivated by the BCS theory, the technique, so-called phonon engineering, was applied to a niobium sheet. The newly emergent high-$T_{c}$ superconductivity, however, cannot be accounted for by the BCS theory. Rather, its exposed configuration, namely a square-lattice oxygen network, is reminiscent of the copper-oxygen plane in cuprate high-$T_c$ superconductors. It turns out that its magnetic result is consistent with the principle of giant atom, which was developed by another heroes of superconductivity, Fritz London and John Slater, in the 1930s, several decades earlier than the propagation of BCS theory. The superconducting transition feature is discussed on the basis of a comprehensive theory of the giant atom -- the theory of hole superconductivity.
A 2D metallic phononic crystal (PnC), which is fabricated by drilling periodic holes in a suspended niobium (Nb) film, is repeatedly cooled and warmed in the temperature range of 2--300 K. During the first five temperature cycles, the resistance of the metallic PnC gradually increases in accordance with the Friedel sum rule, indicating that narrow Nb bridges between adjacent thru-holes are converted into $d$-orbital-filled Mott insulators. The consequent 2D Mott-insulating square lattice is a crystallographic analog of a copper oxide layer in high-temperature superconductors such as YBCO and BSCCO. Subsequent temperature cycles of the thus produced ideal Hubbard crystal realize zero resistance at 60 K, and the zero-resistance state remains up to 300 K, being retained in the atmosphere (i.e., at room temperature, in a terrestrial magnetic field, and under atmospheric pressure). The necessity of the latter temperature cycles remains unclear in this study; however, a possible transition mechanism involving the combined Josephson and charging effects is discussed. The critical current and critical field of the room-temperature superconductor (RTSC) are $I_{C}$ = 18.8 mA and $\mu_{0}H_{\perp}$ = 12 T, respectively. Once a large current exceeding $I_{C}$ is applied to the RTSC, some of the $d$ electrons are forced out, and a special interface consisting of the p-type semiconducting and superconducting states is formed. Carriers diffuse because of the nonequilibrium charge concentration, but they are Andreev-reflected back. In the reversible, thermally isolated system, the entropy never increases, and a simple experiment confirms a low-loss current generated from the interface spontaneously. That is, this study challenges the forbidden perpetual motion machine which can be a solution to the World energy crisis.