Valence-skipping and quasi-two-dimensionality of superconductivity in a van der Waals insulator

Abstract

Valence fluctuation of interacting electrons plays a crucial role in emergent quantum phenomena in correlated electron systems. The theoretical rationale is that this effect can drive a band insulator into a superconductor through charge redistribution around the Fermi level. However, the root cause of such a fluctuating leap in the ionic valency remains elusive. Here, we demonstrate a valence-skipping-driven insulator-to-superconductor transition and realize quasi-two-dimensional superconductivity in a van der Waals insulator GeP under pressure. This is shown to result from valence skipping of the Ge cation, altering its average valency from 3+ to 4+, turning GeP from a layered compound to a three-dimensional covalent system with superconducting critical temperature reaching its maximum of 10 K. Such a valence-skipping-induced superconductivity with a quasi-two-dimensional nature in thin samples, showing a Berezinskii-Kosterlitz-Thouless-like character, is further confirmed by angle-dependent upper-critical-field measurements. These findings provide a model system to examine competing order parameters in valence-skipping systems.

Observation of Emergent Superconductivity in the Topological Insulator Ta2Pd3Te5 via Pressure Manipulation

“…The extracted superconducting gap values Δ g ( T ) are plotted in Figure e as a function of the reduced temperature T / T c , which follows the Bardeen–Cooper–Schrieffer (BCS) theory, …”

“…The well-developed U shape of the d V /d I curves gradually narrow with increasing temperature and eventually become flat around 1.5 K, which is consistent with DC resistance outcomes (Figure h). Additionally, the temperature dependency of the critical current I C was extracted from the d V /d I curves and the value of I C (0) = 0.55 mA can be well-deduced by using the empirical formula (Figure d) , I C ( T ) = I C ( 0 ) ( 1 − ( T / T normalC ) 2 ) …”

“…The well-developed U shape of the d V /d I curves gradually narrow with increasing temperature and eventually become flat around 1.5 K, which is consistent with DC resistance outcomes (Figure h). Additionally, the temperature dependency of the critical current I C was extracted from the d V /d I curves and the value of I C (0) = 0.55 mA can be well-deduced by using the empirical formula (Figure d) , …”

Pressure-driven superconductivity in layered isostructural germanium phosphides

“…Among the considerable family of twodimensional materials, recent examples of binary phosphides showing intrinsic superconductivity have been experimentally reported only in WP [13], Nb 2 P 5 [14] and Mo 3 P [15,16], which are weak-coupling Bardeen-Cooper-Schrieffer (BCS) superconductors. By applying external pressure, superconductivity can be achieved in more binary phosphides, including topological semimetal MoP [17], Weyl semimetal TaP [18], itinerant-electron helimagnet MnP [10,19,20] and valence-skipping semiconductor GeP [21]. These unexpected discoveries of remarkable quantum phenomena open the gate to exploring superconductivity in novel phosphides.…”

Structural phase transformation of quantum spin liquid herbertsmithite via pressure induced enhancement of the cooperative Jahn–Teller effect and antisite disorder

“…It can be utilized to shorten the interatomic distances, regulate the atomic spatial distribution, modulate the electron configuration and exchange interactions, and even induce the emergence of superconductivity or structural phase transitions. 19–22 Modern high-pressure techniques have become a powerful tool that provides a direct and effective route to study the properties of materials by tuning the structural parameters or the crystal structure without changing chemical compositions. Hence the QSL materials have captured increasing attention in the field of high pressure research.…”