In RuCl$_3$, inelastic neutron scattering and Raman spectroscopy reveal a continuum of non-spin-wave excitations that persists to high temperature, suggesting the presence of a spin liquid state on a honeycomb lattice. In the context of the Kitaev mo
del, magnetic fields introduce finite interactions between the elementary excitations, and thus the effects of high magnetic fields - comparable to the spin exchange energy scale - must be explored. Here we report measurements of the magnetotropic coefficient - the second derivative of the free energy with respect to magnetic field orientation - over a wide range of magnetic fields and temperatures. We find that magnetic field and temperature compete to determine the magnetic response in a way that is independent of the large intrinsic exchange interaction energy. This emergent scale-invariant magnetic anisotropy provides evidence for a high degree of exchange frustration that favors the formation of a spin liquid state in RuCl$_3$.
High magnetic fields have revealed a surprisingly small Fermi-surface in underdoped cuprates, possibly resulting from Fermi-surface reconstruction due to an order parameter that breaks translational symmetry of the crystal lattice. A crucial issue co
ncerns the doping extent of this state and its relationship to the principal pseudogap and superconducting phases. We employ pulsed magnetic field measurements on the cuprate HgBa$_2$CuO$_{4+delta}$ to identify signatures of Fermi surface reconstruction from a sign change of the Hall effect and a peak in the temperature-dependent planar resistivity. We trace the termination of Fermi-surface reconstruction to two hole concentrations where the superconducting upper critical fields are found to be enhanced. One of these points is associated with the pseudogap end-point near optimal doping. These results connect the Fermi-surface reconstruction to both superconductivity and the pseudogap phenomena.
In RuCl$_3$, inelastic neutron scattering and Raman spectroscopy reveal a continuum of non-spin-wave excitations that persists to high temperature, suggesting the presence of a spin liquid state on a honeycomb lattice. In the context of the Kitaev mo
del, magnetic fields introduce finite interactions between the elementary excitations, and thus the effects of high magnetic fields - comparable to the spin exchange energy scale - must be explored. Here we report measurements of the magnetotropic coefficient - the second derivative of the free energy with respect to magnetic field orientation - over a wide range of magnetic fields and temperatures. We find that magnetic field and temperature compete to determine the magnetic response in a way that is independent of the large intrinsic exchange interaction energy. This emergent scale-invariant magnetic anisotropy provides evidence for a high degree of exchange frustration that favors the formation of a spin liquid state in RuCl$_3$.
The field of topological electronic materials has seen rapid growth in recent years, in particular with the increasing number of weakly interacting systems predicted and observed to host topologically non-trivial bands. Given the broad appearance of
topology in such systems, it is expected that correlated electronic systems should also be capable of hosting topologically non-trivial states. Interest in correlated platforms is heightened by the prospect that collective behavior therein may give rise to new types of topological states and phenomena not possible in non-interacting systems. However, to date only a limited number of correlated topological materials have been definitively reported due to both the challenge in calculation of their electronic properties and the experimental complexity of correlation effects imposed on the topological aspects of their electronic structure. Here, we report a de Haas-van Alphen (dHvA) study of the recently discovered kagome metal Fe$_3$Sn$_2$ mapping the massive Dirac states strongly coupled to the intrinsic ferromagnetic order. We observe a pair of quasi-two-dimensional Fermi surfaces arising from the massive Dirac states previously detected by spectroscopic probes and show that these band areas and effective masses are systematically modulated by the rotation of the ferromagnetic moment. Combined with measurements of Berry curvature induced Hall conductivity, we find that along with the Dirac fermion mass, velocity, and energy are suppressed with rotation of the moment towards the kagome plane. These observations demonstrate that strong coupling of magnetic order to electronic structure similar to that observed in elemental ferromagnets can be extended to topologically non-trivial electronic systems, suggesting pathways for connecting topological states to robust spintronic technologies.
For both electron- and hole-doped cuprates, superconductivity appears in the vicinity of suppressed broken symmetry order, suggesting that quantum criticality plays a vital role in the physics of these systems. A confounding factor in identifying the
role of quantum criticality in the electron-doped systems is the competing influence of chemical doping and oxygen stoichiometry. Using high quality thin films of Pr$_{2}$CuO$_{4pmdelta}$, we tune superconductivity and uncover the influence of quantum criticality without Ce substitution. We observe magnetic quantum oscillations that are consistent with the presence of small hole-like Fermi surface pockets, and a large mass enhancement near the suppression of superconductivity. Tuning these materials using only oxygen stoichiometry allows the observation of quantum oscillations and provides a new axis with which to explore the physics underlying the electron-doped side of the cuprate phase diagram.
We have studied the angular dependent de Haas-van Alphen oscillations of LaRu$_2$P$_2$ using magnetic torque in pulsed magnetic fields up to 60T. The observed oscillation frequencies are in excellent agreement with the geometry of the calculated Ferm
i surface. The temperature dependence of the oscillation amplitudes reveals effective masses m*($alpha$)=0.71 and m*($beta$)=0.99 m$_e$, which are enhanced over the calculated band mass by $lambda^{cyc}$ of 0.8. We find a similar enhancement $lambda^{gamma} approx 1$ in comparing the measured electronic specific heat ($gamma = 11.5$ mJ/mol K$^2$) with the total DOS from band structure calculations. Remarkably, very similar mass enhancements have been reported in other pnictides LaFe$_2$P$_2$, LaFePO ($T_c approx 4K$), and LaRuPO, independent of whether they are superconducting or not. This is contrary to the common perceptions that the normal state quasi-particle renormalizations reflect the strength of the superconducting paring mechanism and leads to new questions about pairing in isostructural and isoelectronic Ru- and Fe-pnictide superconductors.
We report quantum oscillation measurements that enable the direct observation of the Fermi surface of the low temperature ground state of ba122. From these measurements we characterize the low energy excitations, revealing that the Fermi surface is r
econstructed in the antiferromagnetic state, but leaving itinerant electrons in its wake. The present measurements are consistent with a conventional band folding picture of the antiferromagnetic ground state, placing important limits on the topology and size of the Fermi surface.
