Spinon Fermi Surface Research Articles

Transport properties of a spinon Fermi surface coupled to a U(1) gauge field

With the organic compound $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\text{\ensuremath{-}}{\mathrm{Cu}}_{2}{(\mathrm{C}\mathrm{N})}_{3}$ in mind, we consider a spin liquid system where a spinon Fermi surface is coupled to a U(1) gauge field. Using the nonequilibrium Green's function formalism, we derive the quantum Boltzmann equation for this system. In this system, however, one cannot a priori assume the existence of Landau quasiparticles. We show that even without this assumption, one can still derive a linearized equation for a generalized distribution function. We show that the divergence of the effective mass and of the finite temperature self-energy do not enter these transport coefficients and thus they are well defined. Moreover, using a variational method, we calculate the temperature dependence of the spin resistivity and thermal conductivity of this system.

Susceptibility of a spinon Fermi surface coupled to a U(1) gauge field

We study the theory of a U(1) gauge field coupled to a spinon Fermi surface. Recently this model has been proposed as a possible description of the organic compound $\kappa-(BEDT-TTF)_2 Cu_2 (CN)_3$. We calculate the susceptibility of this system and in particular examine the effect of pairing of the underlying spin liquid. We show that this proposed theory is consistent with the observed susceptibility measurements.

Orbital magnetic field effects in spin liquid with spinon Fermi sea: Possible application toκ−(ET)2Cu2(CN)3

We consider orbital magnetic field effects in a spin liquid phase of a half-filled triangular lattice Hubbard system close to the Mott transition, continuing an earlier exploration of a state with spinon Fermi surface. Starting from the Hubbard model and focusing on the insulator side, we derive an effective spin Hamiltonian up to four-spin exchanges in the presence of magnetic field, and find that the magnetic field couples linearly to spin chirality on the triangles. The latter corresponds to a flux of an internal gauge field in a gauge theory description of the spin liquid, and therefore a static such internal flux is induced. A quantitative estimate of the effect is obtained using a spinon mean field analysis, where we find that this orbital field seen by the spinons is comparable to or even larger than the applied field. We further argue that because the stiffness of the emergent internal gauge field is very small, such a spinon-gauge system is strongly susceptible at low temperatures to an instability of the homogeneous state due to strong Landau level quantization for spinons. This instability is reminiscent of the so-called strong magnetic interaction regime in metals with the usual electromagnetic field, but we estimate that the corresponding temperature--magnetic field range is significantly broader in the spinon-gauge system.

Variational study of triangular lattice spin-1∕2model with ring exchanges and spin liquid state inκ−(ET)2Cu2(CN)3

We study triangular lattice spin-$1∕2$ system with antiferromagnetic Heisenberg and ring exchanges using variational approach focusing on possible realization of spin-liquid states. Trial spin liquid wave functions are obtained by Gutzwiller projection of fermionic mean-field states and their energetics is compared against magnetically ordered trial states. We find that in a range of the ring exchange coupling upon destroying the antiferromagnetic order, the best such spin liquid state is essentially a Gutzwiller-projected Fermi sea state. We propose this spin liquid with a spinon Fermi surface as a candidate for the nonmagnetic insulating phase observed in the organic compound $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{ET})}_{2}{\mathrm{Cu}}_{2}{(\mathrm{CN})}_{3}$, and describe some experimental consequences of this proposal.

Weak magnetism and non-Fermi liquids near heavy-fermion critical points

This paper is concerned with the weak-moment magnetism in heavy-fermion materials and its relation to the non-Fermi liquid physics observed near the transition to the Fermi liquid. We explore the hypothesis that the primary fluctuations responsible for the non-Fermi liquid physics are those associated with the destruction of the large Fermi surface of the Fermi liquid. Magnetism is suggested to be a low-energy instability of the resulting small Fermi surface state. A concrete realization of this picture is provided by a fractionalized Fermi liquid state which has a small Fermi surface of conduction electrons, but also has other exotic excitations with interactions described by a gauge theory in its deconfined phase. Of particular interest is a three-dimensional fractionalized Fermi liquid with a spinon Fermi surface and a U(1) gauge structure. A direct second-order transition from this state to the conventional Fermi liquid is possible and involves a jump in the electron Fermi surface volume. The critical point displays non-Fermi liquid behavior. A magnetic phase may develop from a spin density wave instability of the spinon Fermi surface. This exotic magnetic metal may have a weak ordered moment although the local moments do not participate in the Fermi surface. Experimental signatures of this phase and implications for heavy-fermion systems are discussed.

Absence of U(1) spin liquids in two dimensions

Many popular models of fractionalized spin liquids contain neutral fermionic spinon excitations on a Fermi surface, carrying unit charges under a compact U(1) gauge force. We argue that instanton effects generically render such states unstable to confinement in two spatial dimensions, so that all elementary excitations are gauge neutral, and there is no spinon Fermi surface. Similar results are expected to apply to SU(2) spin liquids. However, fractionalized states can appear when the gauge symmetry is broken down to a discrete subgroup by the Higgs mechanism. Our argument generalizes earlier results on confinement in the pure gauge theory, and on the instability of the U(1) staggered flux and algebraic spin liquids with a Dirac spectrum for fermionic spinons.

The Fermi surface of underdoped high- Tc superconducing cuprates

The coexistence of π-flux state and d-wave RVB state in considered in this paper within the slave boson approach. A critical value of doping concentration δ c is found, below which the coexisting π-flux and d-wave RVB state is favoured in energy. The pseudo Fermi surface of spinons and the physical electron spectral function are calculated. A clear Fermi-level crossing is found along the (0,0) to (π, π) direction, but no such crossing is detected along the (π, 0) to (π, π) direction. Also, an energy gap of d-wave symmetry appears at the Fermi level in our calculation. The above results are in agreement with the angle-resolved photoemission experiments which indicate at a d-wave pseudogap and a half-pocket-like Fermi surface in underdoped cuprates.

Antiferromagnetism of the t-J Model in the Slave Boson Mean Field Approximation

The antiferromagnetic state of the t - J model present in the low doping region in high- T c cuprates is studied in the mean field approximation based on the slave boson framework. It is found that there is a phase transition between localized and itinerant antiferromagnetism depending on the doping rate; the Neel temperature is that of localized spins, i.e., without the Fermi surface of spinons, in the region of very low doping, whereas above critical doping, the onset temperature of the coherent motion of spinons and holons is higher than the Neel temperature. The staggered magnetic moment near absolute zero decreases with doping due to the increasing degree of itineracy, which is consistent with various experiments.