Spin-1/2 Fermions

Nobuo Furukawa, Masatoshi Imada

Spin-fermion model on a square lattice is investigated in the perturbation expansion in terms of the spin-fermion Kondo coupling. The fermion self-energy is calculated in a self-consistent treatment by a dominant pole approximation. The quasi-particle excitation of a doped fermion turns out to be totally incoherent. It suggests that the low energy excitations should be described by the bound state of a fermion and spins as a collective mode and imply that the fixed point of the spin-fermion model with a single fermion is the t - J model. It also suggests the spin-charge separation of the low energy excitations in the light doping region of these two models.

Ki‐Seok Kim

We investigate the quantum phase transition from antiferromagnetism ($AF$) to algebraic spin liquid ($ASL$). {\it We propose that spin 1/2 fermionic spinons in the $ASL$ fractionalize into spin 1/2 bosonic spinons and spinless fermions at the quantum critical point ($QCP$) between the $AF$ and the $ASL$}. Condensation of the bosonic spinons leads to the $AF$, where the condensed bosonic spinons are confined with the spinless fermions to form the fermionic spinons. These fermionic spinons are also confined to make antiferromagnons as elementary excitations in the $AF$. {\it Approaching the $QCP$ from the $AF$, spin 1 critical antiferromagnetic fluctuations are expected to break up into spin 1/2 critical bosonic spinons. Then, these bosonic spinons hybridize with spin 1/2 fermionic spinons, making spinless fermions}. As a result the fermionic spinons decay into the bosonic spinons and the spinless fermions. But, the spinless fermions are confined and thus, only the bosonic spinons emerge at the $QCP$. This coincides with the recent studies of {\it deconfined quantum criticality}\cite{Laughlin_deconfinement,Senthil_deconfinement,Kim1,Ichinose_de confinement}. When the bosonic spinons are gapped, the $ASL$ is realized. The bosonic spinons are confined with the spinless fermions to form the fermionic spinons. These fermionic spinons are deconfined to describe the $ASL$.

Naoum Karchev

We explore a spin-fermion model with fermion-spin-quadrupolar interaction. In a nematic phase, this interaction reduces to a four-fermion interaction that is the basis of superconductivity. When the coupling constant is positive the superconductivity is a p -wave with spin-parallel–paired fermions. When it is negative the superconductivity is a p -wave and fermions are spin-antiparallel paired. For a system with zero chemical potential, even a very small coupling can bind fermions into a bound state that leads to superconductivity. When the chemical potential is non-zero the system possesses quantum critical transition from the normal spin-nematic phase to the phase where superconductivity coexists with spin-nematicity. The value of the quantum critical fermion-spin-nematicity coupling constant depends on the chemical potential.

Franz Schwabl