We point out that in the deep band-inverted state, topological insulators are generically vulnerable against symmetry breaking instability, due to a divergently large density of states of 1D-like exponent near the chemical potential. This feature at
the band edge is associated with a novel van Hove singularity resulting from the development of a Mexican-hat band dispersion. We demonstrate this generic behavior via prototypical 2D and 3D models. This realization not only explains the existing experimental observations of additional phases, but also suggests a route to activate additional functionalities to topological insulators via ordering, particularly for the long-sought topological superconductivities.
The Fermi surface topology of $cI$16 Li at high pressures is studied using a recently developed first-principles unfolding method. We find the occurrence of a Lifshitz transition at $sim$43 GPa, which explains the experimentally observed anomalous on
set of the superconductivity enhancement toward lowered pressure. Furthermore we identify, in comparison with previous reports, additional nesting vectors that contribute to the $cI$16 structural stability. Our study highlights the importance of three-dimensional unfolding analyses for first-principles studies of Fermi surface topologies and instabilities in general.
Realization of conically linear dispersion, termed as Dirac cones, has recently opened up exciting opportunities for high-performance devices that make use of the peculiar transport properties of the massless carriers. A good example of current fashi
on is the heavily studied graphene, a single atomic layered graphite. It not only offers a prototype of Dirac physics in the field of condensed matter and materials science, but also provides a playground of various exotic phenomena. In the meantime, numerous routes have been attempted to search for the next graphene. Despite these efforts, to date there is still no simple guideline to predict and engineer such massless particles in materials. Here, we propose a theoretical recipe to create Dirac cones into anyones favorite materials. The method allows to tailor the properties, such as anisotropy and quantity, in any effective one-band two-dimensional lattice. We demonstrate the validity of our theory with two examples on the square lattice, an unlikely candidate hosting Dirac cones, and show that a graphene-like low-energy electronic structure can be realized. The proposed recipe can be applied in real materials via introduction of vacancy, substitution or intercalation, and also extended to photonic crystal, molecular array, and cold atoms systems.
We demonstrate that the zero-temperature superconducting phase diagram of underdoped cuprates can be quantitatively understood in the strong binding limit, using only the experimental spectral function of the normal pseudo-gap phase without any free
parameter. In the prototypical (La$_{1-x}$Sr$_x$)$_2$CuO$_4$, a kinetics-driven $d$-wave superconductivity is obtained above the critical doping $delta_csim 5.2%$, below which complete loss of superfluidity results from local quantum fluctuation involving local $p$-wave pairs. Near the critical doping, a enormous mass enhancement of the local pairs is found responsible for the observed rapid decrease of phase stiffness. Finally, a striking mass divergence is predicted at $delta_c$ that dictates the occurrence of the observed quantum critical point and the abrupt suppression of the Nernst effects in the nearby region.
We investigate the effect of disordered vacancies on the normal-state electronic structure of the newly discovered alkali-intercalated iron selenide superconductors. To this end we use a recently developed Wannier function based method to calculate f
rom first principles the configuration-averaged spectral function <A(k,w)> of K0.8Fe1.6Se2 with disordered Fe and K vacancies. We find that the disorder can suppress the expected Fermi surface reconstruction without completely destroying the Fermi surface. More interestingly, the disorder effect raises the chemical potential significantly, giving enlarged electron pockets almost identical to highly doped KFe2Se2, without adding carriers to the system.
We report Eu L3-edge resonant inelastic x-ray scattering (RIXS) investigation of the electronic structure of EuB6. We observe that the RIXS spectral weight around 1.1 eV increases dramatically when the system is cooled below the ferromagnetic orderin
g temperature and follows the magnetic order parameter. This spectral feature is attributed to the inter-site excitation from the local 4f orbital to the spin-split 5d orbital on the neighboring site, illustrating the essential role of exchange splitting of the conducting electrons. Based on our density functional theory calculations, the RIXS data suggest that EuB6 at low temperature can be consistently described with a semi-metallic electronic structure with incomplete spin-polarization. We propose routes to achieve half-metallicity in EuB6, which utilize the strong tunability of the electronic structure against gate voltage, strain, and magnetic field.
Despite more than two decades of intensive investigations, the true nature of high temperature (high-$T_c$) superconductivity observed in the cuprates remains elusive to the researchers. In particular, in the so-called `underdoped region, the overall
behavior of superconductivity deviates $qualitatively$ from the standard theoretical description pioneered by Bardeen, Cooper and Schrieffer (BCS). Recently, the importance of phase fluctuation of the superconducting order parameter has gained significant support from various experiments. However, the microscopic mechanism responsible for the surprisingly soft phase remains one of the most important unsolved puzzles. Here, opposite to the standard BCS starting point, we propose a simple, solvable low-energy model in the strong coupling limit, which maps the superconductivity literally into a well-understood physics of superfluid in a special dilute bosonic system of local pairs of doped holes. In the prototypical material (La$_{1-delta}$Sr$_delta$)$_2$CuO$_4$, without use of any free parameter, a $d$-wave superconductivity is obtained for doping above $sim 5.2%$, below which unexpected incoherent $p$-wave pairs dominate. Throughout the whole underdoped region, very soft phases are found to originate from enormous mass enhancement of the pairs. Furthermore, a striking mass divergence is predicted that dictates the occurrence of the observed quantum critical point. Our model produces properties of the superfluid in good agreement with the experiments, and provides new insights into several current puzzles. Owing to its simplicity, this model offers a paradigm of great value in answering the long-standing challenges in underdoped cuprates.
We investigate from first principles the proposed destruction of the controversial eg pockets in the Fermi surface of NaxCoO2 due to Na disorder, by calculating its k-dependent configurational averaged spectral function <A(k,w)>. To this end, a Wanni
er function based method is developed that treats the effects of disorder beyond the mean field. Remarkable spectral broadenings of order ~eV are found for the oxygen orbitals, possibly explaining their absence in the experiments. Contrary to the current lore, however, the eg pockets remain almost perfectly coherent. The developed method is expected to generate exciting opportunities in the study of the countless functional materials that owe their important electronic properties to disordered dopants.
We present an approximative simulation method for quantum many-body systems based on coarse graining the space of the momentum transferred between interacting particles, which leads to effective Hamiltonians of reduced size with the flavor-twisted bo
undary condition. A rapid, accurate, and fast convergent computation of the ground-state energy is demonstrated on the spin-1/2 quantum antiferromagnet of any dimension by employing only two sites. The method is expected to be useful for future simulations and quick estimates on other strongly correlated systems.
