Takahiro Misawa

Spin Fluctuation Theory for Quantum Tricritical Point Arising in Proximity to First-Order Phase Transitions: Applications to Heavy-Fermion Systems, YbRh2Si2, CeRu2Si2, and β-YbAlB4

We propose a phenomenological spin fluctuation theory for antiferromagnetic quantum tricritical point (QTCP), where a first-order phase transition changes into a continuous transition at zero temperature. Under magnetic fields, ferromagnetic quantum critical fluctuations develop around the antiferromagnetic QTCP in addition to antiferromagnetic fluctuations, which is in sharp contrast with the conventional antiferromagnetic quantum critical point. For itinerant electron systems, we show that the temperature dependence of critical magnetic fluctuations around the QTCP is given as χ Q ∝ T -3/2 (χ 0 ∝ T -3/4 ) at the antiferromagnetic ordering (ferromagnetic) wave number q = Q ( q =0). The convex temperature dependence of χ 0 -1 is a characteristic feature of the QTCP, which has never been seen in the conventional spin fluctuation theory. We propose a general theory of quantum tricriticality that has nothing to do with the specific Kondo physics itself, and solves puzzles of quantum criticalities widely obse...

Origin of high- T c superconductivity in doped Hubbard models and their extensions: Roles of uniform charge fluctuations

The doped Hubbard model is a simple model for high-

Ab initio evidence for strong correlation associated with Mott proximity in iron-based superconductors.

{T}_{c}

YbRh2Si2: Quantum Tricritical Behavior in Itinerant Electron Systems

cuprate superconductors, while its ground state remains a challenge. Here, by performing state-of-the-art variational Monte Carlo calculations for the strong-coupling Hubbard model, we find evidence that the

Quantum and Topological Criticalities of Lifshitz Transition in Two-Dimensional Correlated Electron Systems

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Superconductivity and its mechanism in an ab initio model for electron-doped LaFeAsO.

-wave superconducting phase emerges always near the phase separation region and the superconducting order has one-to-one correspondence with the enhancement of charge compressibility. The order as well as the phase separation are vulnerable to realistic intersite Coulomb interaction, while the superexchange interaction enhances both. An appropriate combination of these two widens the stable superconducting phase.

Mott Transition and Phase Diagram of κ-(BEDT-TTF)2Cu(NCS)2 Studied by Two-Dimensional Model Derived from Ab initio Method

We predict that iron-based superconductors discovered near d(6) configuration (5 Fe 3d orbitals filled by 6 electrons) is located on the foot of an unexpectedly large dome of correlated electron matter centered at the Mott insulator at d(5) (namely, half filling). This is based on the many-variable variational Monte Carlo results for ab initio low-energy models derived by the downfolding. The d(5) Mott proximity extends to subsequent emergence of incoherent metals, orbital differentiations due to the Mott physics, and Hunds rule coupling, followed by antiferromagnetic quantum criticality, in quantitative accordance with available experiments.

Magnetic Properties of Ab initio Model of Iron-Based Superconductors LaFeAsO

We propose that the proximity of the first-order transition manifested by the quantum tricritical point (QTCP) explains non-Fermi-liquid properties of YbRh 2 Si 2 . Here, at the QTCP, a continuous phase transition changes into first order at zero temperature. The non-Fermi-liquid behaviors of YbRh 2 Si 2 are veiled in several prominent mysteries; diverging ferromagnetic susceptibility at the antiferromagnetic transition and enhancement of magnetization as well as specific heat. These puzzles are solved by an unconventional criticality derived from our spin fluctuation theory for the QTCP; especially, diverging ferromagnetic susceptibility is quantitatively reproduced.

Quantum Metamagnetic Transitions Induced by Changes in Fermi-Surface Topology: Applications to a Weak Itinerant-Electron Ferromagnet ZrZn2

We study electron correlation effects on quantum criticalities of Lifshitz transitions at zero temperature, using the mean-field theory based on a preexisting symmetry-broken order, in two-dimensio...

Tricritical Behavior in Charge-Order System

Two families of high-temperature superconductors whose critical temperatures are higher than 50 K are known. One are the copper oxides and the other are the iron-based superconductors. Comparisons of mechanisms between these two in terms of common ground as well as distinctions will greatly help in searching for higher T(c) superconductors. However, studies on mechanisms for the iron family based on first principles calculations are few. Here we first show that superconductivity emerges in the state-of-the-art numerical calculations for an ab initio multi-orbital model of an electron-doped iron-based superconductor LaFeAsO, in accordance with experimental observations. Then the mechanism of the superconductivity is identified as enhanced uniform density fluctuations by one-to-one correspondence with the instability towards inhomogeneity driven by first-order antiferromagnetic and nematic transitions. Despite many differences, certain common features with the copper oxides are also discovered in terms of the underlying orbital-selective Mottness found in the iron family.