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Register a new userAnomalous metallic state in the vicinity of Metal to Valence Bond Solid insulator transition in LiVS2
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We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2, known for the formation of valence bond solid (VBS) state out of S = 1 frustrated magnet. LiVS2, which is located at the border between a metal and a correlated insulator, shows a first ordered transition from a paramagnetic metal to a VBS insulator at Tc ~ 305 K upon cooling. The presence of VBS state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in the formation of VBS state. We argue that the high temperature metallic phase of LiVS2 has a pseudo-gap, likely originating from the VBS fluctuation. LiVSe2 was found to be a paramagnetic metal down to 2 K.
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First-order phase transitions in solids are notoriously challenging to study. The combination of change in unit cell shape, long range of elastic distortion, and flow of latent heat leads to large energy barriers resulting in domain structure, hysteresis, and cracking. The situation is still worse near a triple point where more than two phases are involved. The famous metal-insulator transition (MIT) in vanadium dioxide, a popular candidate for ultrafast optical and electrical switching applications, is a case in point. Even though VO2 is one of the simplest strongly correlated materials, experimental difficulties posed by the first-order nature of the MIT as well as the involvement of at least two competing insulating phases have led to persistent controversy about its nature. Here, we show that studying single-crystal VO2 nanobeams in a purpose-built nanomechanical strain apparatus allows investigation of this prototypical phase transition with unprecedented control and precision. Our results include the striking finding that the triple point of the metallic and two insulating phases is at the transition temperature, T_tr = T_c, which we determine to be 65.0 +- 0.1 C. The findings have profound implications for the mechanism of the MIT in VO2, but in addition they demonstrate the importance of such an approach for mastering phase transitions in many other strongly correlated materials, such as manganites and iron-based superconductors.
The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at $approx$ 45 GPa is accompanied by a reentrant Eu valence transition into a $emph{lower}$ valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure.
We have performed Diffusion Quantum Monte Carlo simulations of Li clusters showing that Resonating-Valence-Bond (RVB) pairing correlations between electrons provide a substantial contribution to the cohesive energy. The RVB effects are identified in terms of electron transfers from s- to p-like character, constituting a possible explanation for the breakdown of the Fermi liquid picture observed in recent high resolution Compton scattering experiments for bulk Li.
We present an angle-resolved photoemission study of the electronic structure of the three-dimensional pyrochlore iridate Nd2Ir2O7 through its magnetic metal-insulator transition. Our data reveal that metallic Nd2Ir2O7 has a quadratic band, touching the Fermi level at the Gamma point, similarly to that of Pr2Ir2O7. The Fermi node state is, therefore, a common feature of the metallic phase of the pyrochlore iridates. Upon cooling below the transition temperature, this compound exhibits a gap opening with an energy shift of quasiparticle peaks like a band gap insulator. The quasiparticle peaks are strongly suppressed, however, with further decrease of temperature, and eventually vanish at the lowest temperature, leaving a non-dispersive flat band lacking long-lived electrons. We thereby identify a remarkable crossover from Slater to Mott insulators with decreasing temperature. These observations explain the puzzling absence of Weyl points in this material, despite its proximity to the zero temperature metal-insulator transition.
Elucidating the phase diagram of lattice gauge theories with fermionic matter in 2+1 dimensions has become a problem of considerable interest in recent years, motivated by physical problems ranging from chiral symmetry breaking in high-energy physics to fractionalized phases of strongly correlated materials in condensed matter physics. For a sufficiently large number $N_f$ of flavors of four-component Dirac fermions, recent sign-problem-free quantum Monte Carlo studies of lattice quantum electrodynamics (QED$_3$) on the square lattice have found evidence for a continuous quantum phase transition between a power-law correlated conformal QED$_3$ phase and a confining valence-bond-solid phase with spontaneously broken point-group symmetries. The critical continuum theory of this transition was shown to be the $O(2)$ QED$_3$-Gross-Neveu model, equivalent to the gauged Nambu-Jona-Lasinio model, and critical exponents were computed to first order in the large-$N_f$ expansion and the $epsilon$ expansion. We extend these studies by computing critical exponents to second order in the large-$N_f$ expansion and to four-loop order in the $epsilon$ expansion below four spacetime dimensions. In the latter context, we also explicitly demonstrate that the discrete $mathbb{Z}_4$ symmetry of the valence-bond-solid order parameter is dynamically enlarged to a continuous $O(2)$ symmetry at criticality for all values of $N_f$.
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