We examine recent magnetic torque measurements in two compounds, $gamma$-Li$_2$IrO$_3$ and RuCl$_3$, which have been discussed as possible realizations of the Kitaev model. The analysis of the reported discontinuity in torque, as an external magnetic
field is rotated across the $c-$axis in both crystals, suggests that they have a translationally-invariant chiral spin-order of the from $<{bf S}_i. ({bf S}_j ~times ~ {bf S}_k)> e 0$ in the ground state and persisting over a very wide range of magnetic field and temperature. An extra-ordinary $|B|B^2$ dependence of the torque for small fields, beside the usual $B^2$ part, is predicted due to the chiral spin-order, and found to be consistent with experiments upon further analysis of the data. Other experiments such as inelastic scattering and thermal Hall effect and several questions raised by the discovery of chiral spin-order, including its topological consequences are discussed.
Half a century ago, Mott noted that tuning the carrier density of a semimetal towards zero produces an insulating state in which electrons and holes form bound pairs. It was later argued that such pairing persists even if a semiconducting gap opens i
n the underlying band structure, giving rise to what has become known as the strong coupling limit of an `excitonic insulator. While these `weak and `strong coupling extremes were subsequently proposed to be manifestations of the same excitonic state of electronic matter, the predicted continuity of such a phase across a band gap opening has not been realized experimentally in any material. Here we show the quantum limit of graphite, by way of temperature and angle-resolved magnetoresistance measurements, to host such an excitonic insulator phase that evolves continuously between the weak and strong coupling limits. We find that the maximum transition temperature T_EI of the excitonic phase is coincident with a band gap opening in the underlying electronic structure at B_0= 46 +/- 1 T, which is evidenced above T_EI by a thermally broadened inflection point in the magnetoresistance. The overall asymmetry of the observed phase boundary around B_0 closely matches theoretical predictions of a magnetic field-tuned excitonic insulator phase in which the opening of a band gap marks a crossover from predominantly momentum-space pairing to real-space pairing.
Some of the most remarkable phenomena---and greatest theoretical challenges---in condensed matter physics arise when $d$ or $f$ electrons are neither fully localized around their host nuclei, nor fully itinerant. This localized/itinerant duality unde
rlies the correlated electronic states of the high-$T_c$ cuprate superconductors and the heavy-fermion intermetallics, and is nowhere more apparent than in the $5f$ valence electrons of plutonium. Here we report the full set of symmetry-resolved elastic moduli of $PuCoGa_5$---the highest $T_c$ superconductor of the heavy fermions ($T_c$=18.5 K)---and find that the bulk modulus softens anomalously over a wide range in temperature above $T_c$. Because the bulk modulus is known to couple strongly to the valence state, we propose that plutonium valence fluctuations drive this elastic softening. This elastic softening is observed to disappear when the superconducting gap opens at $T_c$, suggesting that plutonium valence fluctuations have a strong footprint on the Fermi surface, and that $PuCoGa_5$ avoids a valence-transition by entering the superconducting state. These measurements provide direct evidence of a valence instability in a plutonium compound, and suggest that the unusually high-$T_c$ in this system is driven by valence fluctuations.
The highest superconducting transition temperatures in the cuprates are achieved in bilayer and trilayer systems, highlighting the importance of intralayer interactions for high Tc. It has been argued that interlayer hybridization vanishes along the
nodal directions by way of a specific pattern of orbital overlap. Recent quantum oscillation measurements in bilayer cuprates have provided evidence for a residual bilayer-splitting at the nodes that is sufficiently small to enable magnetic breakdown tunneling at the nodes. Here we show that several key features of the experimental data can be understood in terms weak spin-orbit interactions naturally present in bilayer systems, whose primary effect is to cause the magnetic breakdown to be accompanied by a spin flip. These features can now be understood include the equidistant set of three quantum oscillation frequencies, the asymmetry of the quantum oscillation amplitudes in c-axis transport compared to ab-plane transport, and the anomalous magnetic field angle dependence of the amplitude of side frequencies suggestive of small effective g-factors. We suggest that spin-orbit interactions in bilayer systems can further affect the structure of the nodal quasiparticle spectrum in the superconducting phase.
We report magneto-transport studies of topological insulator Bi_{2}Te_{3} thin films grown by pulsed laser deposition. A non-saturating linear-like magneto-resistance (MR) is observed at low temperatures in the magnetic field range from a few Tesla u
p to 60 Tesla. We demonstrate that the strong linear-like MR at high field can be well understood as the weak antilocalization phenomena described by Hikami-Larkin-Nagaoka theory. Our analysis suggests that in our system, a topological insulator, the elastic scattering time can be longer than the spin-orbit scattering time. We briefly discuss our results in the context of Dirac Fermion physics and quantum linear magnetoresistance.
In type-II superconductors that contain a lattice of magnetic moments, vortices polarize the magnetic system inducing additional contributions to the vortex mass, vortex viscosity, and vortex-vortex interaction. Extra magnetic viscosity is caused by
radiation of spin waves by a moving vortex. Like in the case of Cherenkov radiation, this effect has a characteristic threshold behavior and the resulting vortex viscosity may be comparable to the well-known Bardeen-Stephen contribution. The threshold behavior leads to an anomaly in the current-voltage characteristics, and a drop in dissipation for a current interval that is determined by the magnetic excitation spectrum.
The loop-current state discovered in the pseudogap phase of cuprates breaks time reversal symmetry and lowers the point group symmetry of the crystal. The order parameter and the magnetic structure within each unit cell which is associated with it ca
n be described by a toroidal moment parallel to the copper-oxide planes. We discuss lattice point group symmetry of the magnetic structure. As an application, we discuss a few effects that necessarily accompany order parameter in the pseudogap phase. The magnitude estimated for these specific effects makes them hard to observe because they rely on the small magnetic fields associated with the order parameter. Effects, associated with the electronic energies are much larger. Some of them have already been discussed.
Marginal Fermi liquid was originally introduced as a phenomenological description of the cuprates in a part of the metallic doping range which appears to be governed by fluctuations due to a quantum-critical point. An essential result due to the form
of the assumed fluctuation spectra is that the large inelastic quasiparticle relaxation rate near the Fermi-surface is proportional to the energy measured from the chemical potential, $tau_i^{-1}proptoepsilon$. We present a microscopic long-wavelength derivation of the hydrodynamic properties in such a situation by an extension of the procedure that Eliashberg used for the derivation of the hydrodynamic properties of a Landau-Fermi-liquid. In particular, the density-density and the current-current correlations and the relation between the two are derived, and the connection to microscopic calculations of the frequency dependence of the optical conductivity with an additional fermi-liquid correction factor shown to follow. The method used here may be necessary, quite generally, for the correct hydrodynamic theory for any problem of quantum-critical fluctuations in fermions.
We consider the non-analytic terms in the spin susceptibility arising as a result of rescaterring of pairs of quasiparticles. We emphasize the importance of rescattering in the Cooper channel for the analysis of the temperature dependences in the two
-dimensional electron systems in the ballistic regime. In the calculation of the linear in $T$ term we use angular harmonics in the Cooper channel, because for each harmonic the interaction amplitude is renormalized independently. We observe, that as a consequence of strong renormalizations in the Cooper ladder, the temperature derivative of the spin susceptibility may change its sign at low temperatures.
