Ultrathin two-dimensional superconductivity with strong spin–orbit coupling

Abstract

We report on a study of epitaxially grown ultrathin Pb films that are only a few atoms thick and have parallel critical magnetic fields much higher than the expected limit set by the interaction of electron spins with a magnetic field, that is, the Clogston-Chandrasekhar limit. The epitaxial thin films are classified as dirty-limit superconductors because their mean-free paths, which are limited by surface scattering, are smaller than their superconducting coherence lengths. The uniformity of superconductivity in these thin films is established by comparing scanning tunneling spectroscopy, scanning superconducting quantum interference device (SQUID) magnetometry, double-coil mutual inductance, and magneto-transport, data that provide average superfluid rigidity on length scales covering the range from microscopic to macroscopic. We argue that the survival of superconductivity at Zeeman energies much larger than the superconducting gap can be understood only as the consequence of strong spin-orbit coupling that, together with substrate-induced inversionsymmetry breaking, produces spin splitting in the normal-state energy bands that is much larger than the superconductor's energy gap.superconductivity is a topic of growing interest in contemporary condensed matter physics. Early experimental work in this field used granular thin films to study phase transitions to insulating normal states driven by weakened superfluid rigidity in the ultrathin film regime. Recent experimental progress (1-12) in epitaxial growth of uniform 2D superconductors whose properties are largely intrinsic has opened up new possibilities for the design of superconducting systems with specific desirable physical properties. Indeed, these almost ideal 2D systems have yielded (6, 12, 13) surprisingly robust superconductivity in films that are only one or two atomic layers thick, and very recently the observation of an astonishingly high T c in singlelayer FeSe on SrTiO 3 (14-17). In addition, because 2D superconductors must be placed on a substrate, they necessarily have broken inversion symmetry and Rashba-type spin-orbit interactions that break the spin degeneracy of quasiparticle levels in the normal state, and enable the possibility of achieving topological (18-21) superconducting states.Here, we investigate the superconducting properties of strong spin-orbit coupling 2D superconductors using epitaxially grown, ultrathin Pb films on Si. By establishing that the superfluid rigidity vanishes at essentially the same T c when measured on different length scales, from atomic to macroscopic (greater than millimeter), we demonstrate the uniformity of the superconductivity in our films and obtain highly reliable quantitative superfluid density (SFD) values. We then perform magneto-transport measurements in parallel fields, which clearly establish that the Clogston-Chandrasekhar (CC) limit does not apply to our films. Superconductivity at Zeeman fields well in excess of the superconducting energy gap can be understood only as a consequence of strong...

2D superconductivity and vortex dynamics in 1T-MoS2

“…In our sample, the measured B IK ∥ at 12 mK is ~ 6.25 T, ~four-times the value 1.618 T predicted by the Pauli limit. A similar enhancement in the B IK ∥ has been observed in systems with high SOC (i) TMDC superconductors in the monolayer limit owing to Ising pairing of electrons [29][30][31] (ii) crystalline thin-film superconductors with high SOC and substrate induced symmetry breaking 25 and, (iii) recently in centro-symmetrtric few layer TMDCs 32 . This has been suggested due to the SOC induced spin splitting of conduction electrons making pair breaking unfavorable under external magnetic field; further studies are required to understand the detailed mechanism.…”

“…In systems with low disorder, a 2D quantum metallic phase, the Bose metal, is emerged due to magnetic field induced gauge fluctuations [22][23][24] , as opposed to an insulating state observed in strongly disordered systems. Until the advent of vW materials 3 2D superconductivity has been confined to disordered material systems 19,[25][26][27][28] such as interfaces of bulk materials and thin films. Low structural disorders and high crystallinity makes 2D superconductivity and associated phenomena easier to observe in vW systems compared to disordered 2D systems.…”

Chirality blockade of Andreev reflection in a magnetic Weyl semimetal

“…1, a magnetic field in the y direction, in the plane of the NS interface and perpendicular to the magnetization, activates Andreev reflection when the Zeeman energy E Zeeman becomes comparable to the superconducting gap 0 . (To prevent pair-breaking effects from this Zeeman field, one can use a thin-film superconductor with strong spin-orbit coupling [37].) For a typical Zeeman energy of 1 meV/T and a typical gap of 0.1 meV, a 100 mT magnetic field can then activate the transfer of Cooper pairs through the NS interface.…”

Pressure-Induced Large Volume Collapse, Plane-to-Chain, Insulator to Metal Transition in CaMn2Bi2

“…Initially it was believed that quantum fluctuations can only be appreciable in magnetic materials where magnetic frustration is present, [7][8][9][10][11] however, it is now realized that strong spin-orbit coupling (SOC) can also produce the energies needed for quantum fluctuations. [12][13][14] As the heaviest non-radioactive element, bismuth, in main group V, is located at the Zintl border in the periodic table. [15][16][17][18][19][20] Bi has moderate electronegativity (and can commonly act as either an anion or a cation) and strong spin-orbit coupling.…”