Spin-orbit coupling has proven indispensable in realizing topological materials and more recently Ising pairing in two-dimensional superconductors. This pairing mechanism relies on inversion symmetry breaking and sustains anomalously large in-plane polarizing magnetic fields whose upper limit is expected to diverge at low temperatures, although experimental demonstration of this has remained elusive due to the required fields. In this work, the recently discovered superconductor few-layer stanene, i.e. epitaxially strained -Sn, is shown to exhibit a new type of Ising pairing between carriers residing in bands with different orbital indices near the Γ-point. The bands are split as a result of spin-orbit locking without the participation of inversion symmetry breaking. The in-plane upper critical field is strongly enhanced at ultra-low temperature and reveals the sought for upturn.

The motion of electrons in a solid has a profound effect on its topological properties and may result in a nonzero Berry's phase, a geometric quantum phase encoded in the system's electronic wave function. Despite its ubiquity, there are few experimental observations of Berry's phase of bulk states. Here, we report detection of a nontrivial π Berry's phase in the bulk Rashba semiconductor BiTeI via analysis of the Shubnikov-de Haas (SdH) effect. The extremely large Rashba splitting in this material enables the separation of SdH oscillations, stemming from the spin-split inner and outer Fermi surfaces. For both Fermi surfaces, we observe a systematic π-phase shift in SdH oscillations, consistent with the theoretically predicted nontrivial π Berry's phase in Rashba systems.

Quantum transport of quasi–two-dimensional Dirac fermions is largely controlled by magnetic order in a layered magnet.

In magnetically controllable ferroelectrics 1-3 , electric polarization is induced by charge redistribution or lattice distortions that occur to minimize the energy associated with both the magnetic order and interaction of spins with an applied magnetic field. Conventional approaches to designing materials that exploit such spin-mediated behaviour have focused mainly on developing the cycloidal spin order 4,5 , and thereby producing ferroelectric behaviour through the so-called antisymmetric Dzyaloshinskii-Moriya interaction 6-8. However, engineering such spin structures is challenging. Here we suggest a different approach. Direct measurements of magnetic-field-dependent variations in the polarization of the one-dimensional organic quantum magnet, tetrathiafulvalene-p-bromanil, suggest a spin-Peierls instability has an important role in its response. Our results imply that one-dimensional quantum magnets, such as organic charge-transfer complexes, could be promising candidates in the development of magnetically controllable ferroelectric materials. A straightforward guideline for designing spin-driven ferroelectricity might be to use the symmetric interaction, simply because it is more dominant in ubiquitous magnets, rather than the antisymmetric exchange interaction (that is, the Dzyaloshinskii-Moriya interaction). In this context, the up-up-down-down (↑↑↓↓) magnetic order on the alternating (ABAB) atom sites was proposed as a promising candidate for symmetric-exchange-driven ferroelectricity 9 ; this prediction has actually been confirmed in the frustrated Ising chain system, Ca 3 Co 2−x Mn x O 6 (A = Co and B = Mn) 10 , and the f-d spin-coupled state of GdFeO 3 (A = Gd and B = Fe) 11. Moreover, there is a theoretical argument that symmetric-exchange-driven ferroelectricity can host potentially large polarization 12. This recent progress is prompting a pursuit of not only materials showing the ↑↑↓↓ order but also other clear-cut examples of symmetric-exchange-driven ferroelectricity. In the same way as one-dimensional (1D) metals with inherent lattice instability (Peierls instability) 13 , 1D Heisenberg spin-1/2 quantum magnets possess an instability to form a dimer-singlet state because of the energy gain of symmetric exchange 14,15. This is known as the spin-Peierls instability, a textbook example of spin-lattice coupling. Therefore, the spin-Peierls instability with alternating ABAB spin sites can host a polar singlet-dimer and hence is expected to provide a new mechanism for magnetically controllable ferroelectrics of symmetric-exchange origin. The organic charge-transfer salt TTF-BA (tetrathiafulvalene-p-bromanil) has been proposed as a possible candidate of this new class 16 of material, namely a ferroelectric spin-Peierls material; nevertheless, direct observations of ferroelectricity, that is, electric-field reversal of polarization, have LETTERS NATURE PHYSICS

We have studied the magnetization of the recently discovered heavy fermion superconductor UTe 2 up to 56 T in pulsed-magnetic fields. A first-order metamagnetic transition has been clearly observed at H m =34.9 T when the magnetic field H is applied along the orthorhombic hard-magnetization b-axis. The transition has a critical end point at ∼11 K and 34.8 T, where the first order transition terminates and changes into a crossover regime. Using the thermodynamic Maxwell relation, we have evaluated the field dependence of the Sommerfeld coefficient of the specific heat directly related to the superconducting pairing. From the analysis, we found a significant enhancement of the effective mass centered at H m , which is reminiscent of the field-reentrant superconductivity of the ferromagnet URhGe in transverse fields. We discuss the origin of their field-robust superconductivity.