Youhei Yamaji

First-Principles Study of the Honeycomb-Lattice Iridates Na 2 IrO 3 in the Presence of Strong Spin-Orbit Interaction and Electron Correlations

An effective low-energy Hamiltonian of itinerant electrons for iridium oxide Na2IrO3 is derived by an ab initio downfolding scheme. The model is then reduced to an effective spin model on a honeycomb lattice by the strong coupling expansion. Here we show that the ab initio model contains spin-spin anisotropic exchange terms in addition to the extensively studied Kitaev and Heisenberg exchange interactions, and allows us to describe the experimentally observed zigzag magnetic order, interpreted as the state stabilized by the antiferromagnetic coupling of the ferromagnetic chains. We clarify possible routes to realize quantum spin liquids from existing Na2IrO3.

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...

Restricted Boltzmann machine learning for solving strongly correlated quantum systems

We develop a machine learning method to construct accurate ground-state wave functions of strongly interacting and entangled quantum spin as well as fermionic models on lattices. A restricted Boltzmann machine algorithm in the form of an artificial neural network is combined with a conventional variational Monte Carlo method with pair product (geminal) wave functions and quantum number projections. The combination allows an application of the machine learning scheme to interacting fermionic systems. The combined method substantially improves the accuracy beyond that ever achieved by each method separately, in the Heisenberg as well as Hubbard models on square lattices, thus proving its power as a highly accurate quantum many-body solver.

Metallic Interface Emerging at Magnetic Domain Wall of Antiferromagnetic Insulator: Fate of Extinct Weyl Electrons

Topological insulators, in contrast to ordinary semiconductors, accompany protected metallic surfaces described by Dirac-type fermions. Here, we theoretically show another emergent two-dimensional metal embedded in the bulk insulator is realized at a magnetic domain wall. The domain wall has long been studied as ingredients of both old-fashioned and leading-edge spintronics. The domain wall here, as an interface of seemingly trivial antiferromagnetic insulators, emergently realizes a functional interface preserved by zero modes with robust two-dimensional Fermi surfaces, where pyrochlore iridium oxides proposed to host condensed-matter realization of Weyl fermions offer such examples at low temperatures. The existence of ingap states pinned at domain walls, theoretically resembling spin/charge solitons in polyacetylene, and protected as the edge of hidden one-dimensional weak Chern insulators characterized by a zero-dimensional class A topological invariant, solves experimental puzzles observed in R2Ir2O7 with rare earth elements R. The domain wall realizes a novel quantum confinement of electrons and embosses a net uniform magnetization, which enables magnetic control of electronic interface transports beyond semiconductor paradigm.

Topological Insulators from Spontaneous Symmetry Breaking Induced by Electron Correlation on Pyrochlore Lattices

We study an extended Hubbard model with the nearest-neighbor Coulomb interaction on the pyrochlore lattice at half filling. An interaction-driven insulating phase with nontrivial Z 2 invariants emerges at the Hartree–Fock mean-field level in the phase diagram. This topological insulator phase competes with other ordered states and survives in a parameter region surrounded by a semimetal, antiferromagnetic and charge ordered insulators. The symmetries of these phases are group-theoretically analyzed. We also show that the ferromagnetic interaction enhances the stability of the topological phase.

Mott physics on helical edges of two-dimensional topological insulators

We study roles of electron correlations on topological insulators on the honeycomb lattice with the spin-orbit interaction. Accurate variational Monte Carlo calculations show that the increasing on-site Coulomb interactions cause a strong suppression of the charge Drude weight in the helical-edge metallic states leading to a presumable Mott transition from a conventional topological insulator to an edge Mott insulator before a transition to a bulk antiferromagnetic insulator. The intermediate bulk-topological and edge-Mott-insulator phase has a helical spin-liquid character with the protected time-reversal symmetry.

YbRh2Si2: Quantum Tricritical Behavior in Itinerant Electron Systems

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

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...

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

We clarify that metamagnetic transitions in three dimensions show unusual properties as quantum phase transitions if they are accompanied by changes in Fermi-surface topology. An unconventional universality deeply affected by the topological nature of Lifshitz-type transitions emerges around the marginal quantum critical point (MQCP). Here the MQCP is defined by the meeting point of the finite temperature critical line and a quantum critical line running on the zero temperature plane. The MQCP offers a marked contrast with the Ising universality and the gas-liquid-type criticality satisfied for conventional metamagnetic transitions. At the MQCP, the inverse magnetic susceptibility chi^-1 has diverging slope as a function of the magnetization m (namely, | d chi^-1/d m | ->infty) in one side of the transition, which should not occur in any conventional quantum critical phenomena. The exponent of the divergence can be estimated even at finite temperatures. We propose that such an unconventional universality indeed accounts for the metamagnetic transition in ZrZn_2.We clarify that metamagnetic transitions in three dimensions show unusual properties as quantum phase transitions if they are accompanied by changes in Fermi-surface topology. An unconventional uni...

Composite-Fermion Theory for Pseudogap, Fermi Arc, Hole Pocket, and Non-Fermi Liquid of Underdoped Cuprate Superconductors

We propose that an extension of the exciton concept to doped Mott insulators offers a fruitful insight into challenging issues of the copper oxide superconductors. In our extension, new fermionic excitations called cofermions emerge in conjunction to generalized excitons. The cofermions hybridize with conventional quasiparticles. Then a hybridization gap opens, and is identified as the pseudogap observed in the underdoped cuprates. The resultant Fermi-surface reconstruction naturally explains a number of unusual properties of the underdoped cuprates, such as the Fermi arc and/or pocket formation.