The magnetic field response of the Mott-insulating honeycomb iridate Na2IrO3 is investigated using torque magnetometry measurements in magnetic fields up to 60 tesla. A peak-dip structure is observed in the torque response at magnetic fields corresponding to an energy scale close to the zigzag ordering (≈ 15K) temperature. Using exact diagonalization calculations, we show that such a distinctive signature in the torque response constrains the effective spin models for these classes of Kitaev materials to ones with dominant ferromagnetic Kitaev interactions, while alternative models with dominant antiferromagnetic Kitaev interactions are excluded. We further show that at high magnetic fields, long range spin correlation functions decay rapidly, signaling a transition to a long-sought-after field-induced quantum spin liquid beyond the peak-dip structure. Kitaev systems are thus revealed to be excellent candidates for field-induced quantum spin liquids, similar physics having been suggested in another Kitaev material α−RuCl3.
We study the effect of Hund's splitting of repulsive interactions on electronic phase transitions in the multiorbital topological crystalline insulator Pb1−xSnxTe, when the chemical potential is tuned to the vicinity of low-lying Type-II Van Hove singularities. Nontrivial Berry phases associated with the Bloch states impart momentum-dependence to electron interactions in the relevant band. We use a multipatch parquet renormalization group (RG) analysis for studying the competition of different electronic phases, and find that if the dominant fixed-point interactions correspond to antiparallel spin configurations, then a chiral p-wave Fulde-Ferrell-Larkin-Ovchinnikov(FFLO) state is favored, otherwise, none of the commonly encountered electronic instabilities occur within the one-loop parquet RG approach.Topological crystalline insulators (TCIs) have lowenergy surface states in certain high symmetry directions, protected by crystalline symmetry [1]. Unlike conventional Z 2 topological insulators [2][3][4][5], the nature of these low-energy states is sensitive to the surface orientation. In particular, it has been shown in the recently discovered TCI Pb 1−x Sn x Te [6-9] that the band structure of the (001) surface allows for the presence of Type-II Van Hove singularities [10], with a diverging density of states, which opens up the possibility of a variety of competing Fermi-surface instabilities brought about by weak repulsive interparticle interactions [11][12][13][14][15]. In particular, the parquet approximation for studying competing phases in a system with multiple Fermi pockets has proved very useful in the context of unconventional superconductivity [16][17][18] in cuprates [19], graphene [20] and semimetal thin films [21]. However, in a multiorbital system like Pb 1−x Sn x Te, phase competition needs to be studied taking into account the effect of Hund's splitting of interactions. The importance of Hund's coupling has generally been underemphasized in parquet renormalization group analyses of multiorbital systems for reasons of convenience, but recent developments show that Hund's coupling may play an important role in electronic instabilities of multiorbital systems [22,23].In this paper, we employ a multipatch parquet renormalization group (RG) analysis including Hund's splitting effects, and show that even relatively small amounts of Hund's splitting can have a dramatic effect on the very existence of electronic instabilities on the surface of Pb 1−x Sn x Te. Depending on the sign of the Hund's splitting, we find that away from perfect nesting, either a chiral p-wave FFLO [24,25] state is stabilized or none of the commonly encountered electronic instabilities occur at the level of the one-loop parquet approach. A characteristic feature of Pb 1−x Sn x Te is that the surface bands are effectively spinless, which rules out s-wave pairing, that would otherwise prevail over p-wave pairing in the presence of nonmagnetic disorder [26][27][28].The topological crystalline insulator surface that we consider offer...
We study the effect of a uniform external magnetization on p-wave superconductivity on the (001) surface of the crystalline topological insulator(TCI) Pb1−xSnxTe. It was shown by us in an earlier work that a chiral p-wave finite-momentum pairing (FFLO) state can be stabilized in this system in the presence of weak repulsive interparticle interactions. In particular, the superconducting instability is very sensitive to the Hund's interaction in the multiorbital TCI, and no instabilities are found to be possible for the "wrong" sign of the Hund's splitting. Here we show that for a finite Hund's splitting of interactions, a significant value of the external magnetization is needed to degrade the surface superconductivity, while in the absence of the Hund's interaction, an arbitrarily small external magnetization can destroy the superconductivity. This implies that multiorbital effects in this system play an important role in stabilizing electronic order on the surface.
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