Spin-charge Gauge Approach Research Articles

Charge carriers with fractional exclusion statistics in cuprates

We show that in the SU(2)XU(1) spin-charge gauge approach we developed earlier one can attribute consistently an exclusion statistics with parameter 1/2 to the spinless charge carriers of the t-J model in two dimensions(2D), as it occurs in one dimension (1D). Like the 1D case, the no-double occupation constraint is at the origin of this fractional exclusion statistics. With this statistics we recover a "large" Fermi volume of holes at high dopings, close to that of the tight binding approximation. Furthermore, the composite nature of the hole, made of charge and spin carriers only weakly bounded, can provide a natural explanation of many unusual experimental features of the hole-doped cuprates.

Superfluid Density in Cuprates: Hints on Gauge Compositeness of the Holes

We show that several features (the three-dimensional XY universality for moderate underdoping, the almost-BCS behaviour for moderate overdoping and the critical exponent) of the superfluid density in hole-doped cuprates hint at a composite structure of the holes. This idea can be implemented in a spin-charge gauge approach to the $t - t' - J$ model and provides indeed good agreement with available experimental data.

Universality in Cuprates: A Gauge Approach

In high-$T_c$ cuprates many quantities exhibit a non-Fermi liquid universality hinting at a very peculiar structure of the underlying pairing mechanism for superconductivity: in this work we focus on the universality for the in-plane resistivity and for the superfluid density. We outline the previously developed spin-charge gauge approach to superconductivity in hole-doped cuprates: we decompose the hole of the $t-t'-J$ model for the $\mathrm{Cu}\mathrm{O}_2$ planes as the product of a spinful, chargeless gapped spinon and of a spinless, charged holon with Fermi surface. Each one of these particle excitations is bound to a statistical gauge flux, allowing one to optimize their statistics. We show that this model allows for a natural interpretation of the universality: within this approach, under suitable conditions, the spinonic and holonic contributions to a response function sum up according to a Ioffe-Larkin rule. We argue that, if the spinonic contribution dominates, then one should expect strongly non-Fermi-liquid-like universality, due to the insensitivity of spinons to Fermi surface details. The in-plane resistivity and superfluid density are indeed dominated by the spinons in the underdoped region. We theoretically derive these quantities, discussing their universal behaviours and comparing them with experimental data.

Gauge approach to the ‘pseudogap’ phenomenology of the spectral weight in high Tc cuprates

We assume the t–t′–J model to describe the CuO2 planes of hole-doped cuprates and we adapt the spin–charge gauge approach, previously developed for the t–J model, to describe the holes in terms of a spinless fermion carrying the charge (holon) and a neutral boson carrying spin 1/2 (spinon), coupled by a slave-particle gauge field. In this framework we consider the effects of a finite density of incoherent holon pairs in the normal state. Below a crossover temperature, identified as the experimental ‘upper pseudogap’, the scattering of the ‘quanta’ of the phase of the holon-pair field against holons reproduces the phenomenology of nodal Fermi arcs coexisting with a gap in the antinodal region. We thus obtain a microscopic derivation of the main features of the hole spectra due to the pseudogap. This result is obtained through a holon Green function which follows naturally from the formalism and analytically interpolates between a Fermi liquid-like and a d-wave superconductor behaviour as the coherence length of the holon-pair order parameter increases. By inserting the gauge coupling with the spinon we construct explicitly the hole Green function and calculate its spectral weight and the corresponding density of states. So we prove that the formation of holon pairs induces a depletion of states on the hole Fermi surface. We compare our results with ARPES and tunnelling experimental data. In our approach the hole preserves a finite Fermi surface until the superconducting transition, where it reduces to four nodes. Therefore we propose that the gap seen in the normal phase of cuprates is due to the thermal broadening of the SC-like peaks masking the Fermi-liquid peak in the spectral weight. The Fermi arcs then correspond to the region of the Fermi surface where the Fermi-liquid peak is unmasked.

Superconductivity from pseudogap through attraction between spin-vortices

Within the framework of the spin–charge gauge approach to the 2D t–J model (Marchetti et al., 2007 [1]) we propose a new non-BCS mechanism for superconductivity in hole-underdoped cuprates. The gluing force is an attraction between spin-vortices centered at the empty sites (described by fermionic holons) in two different Néel sublattices. This attraction induces in turn, through a gauge interaction originated from the no-double occupation constraint, formation of resonance valent bond (RVB) spin pairs (described in terms of bosonic spinons). When holon pairs condense, a finite density of RVB spinon pairs appears. Superconductivity occurs at T=0 at finite doping concentration, when both the holon-vortex pairs and the RVB spinon pairs are condensed. Its precursor is a normal state region where magnetic vortices proliferate, leading to superconductivity via a 3D-gauged-XY transition.

Gauge approach to the specific heat in the normal state of cuprates

Many experimental features of the electronic specific heat and entropy of high-${T}_{c}$ cuprates in the normal state, including the nontrivial temperature dependence of the specific-heat coefficient $\ensuremath{\gamma}$ and the negative intercept of the extrapolated entropy to $T=0$ for underdoped cuprates, are reproduced using the spin-charge gauge approach to the $t\text{\ensuremath{-}}J$ model. The entropy turns out to be basically due to fermionic excitations but with a temperature dependence of the specific-heat coefficient controlled by fluctuations of a gauge field coupling them to gapful bosonic excitations. In particular the negative intercept of the extrapolated entropy at $T=0$ in the pseudogap ``phase'' is attributed to the scalar component of the gauge field, which implements the local no-double occupancy constraint.

Spin–charge gauge compositeness of electron in HTS: Hints from metal–insulator crossover and entropy behavior

We show that the coexistence of Fermi arcs and metal–insulator crossover of the in-plane resistivity can give a hint of a peculiar “gauge compositeness” of the electron in hole-doped high T c cuprates and a similar hint also comes from the negative intercept at T = 0 of the electronic entropy extrapolated from moderate temperatures in the “pseudogap phase”. An implementation of this “compositeness” within the spin–charge gauge approach is outlined and is employed to discuss the above phenomena.

Transport properties of HTS cuprates via spin–charge gauge approach

We show that the spin–charge gauge approach to the 2D t– J model can naturally explain many experimental features of transport properties for HTS cuprates, in particular temperature crossover phenomena in the “pseudogap phase” (PG) and 1/ T behavior of conductivities in the “strange metal phase” (SM) at higher T or x (doping concentration).

Spin-charge gauge approach to metal–insulator crossover and transport properties inhigh-Tc cuprates

The spin-charge gauge approach to the metal–insulatorcrossover (MIC) and other anomalous transport properties inhigh-Tc cuprates isbriefly reviewed. A U(1) field gauging the global charge symmetry and anSU(2) field gauging the global spin–rotational symmetry are introduced to study the two-dimensionalt–J model in the limit . The MIC, as a clue to the understanding of the ‘pseudogap’ (PG) phase, isattributed to the competition between the short-range antiferromagnetic order anddissipative motion of charge carriers coupled to the slave-particle gauge field.The composite particle formed by binding the charge carrier (holon) and spinexcitation (spinon) via the slave-particle gauge field exhibits a number of peculiarproperties, and the calculated results are in good agreement with experimentaldata for both PG and ‘strange metal’ phases. Connections to other gauge fieldapproaches in studying the strong correlation problem are also briefly outlined.

Transport properties in the strange-metal phase of high-Tccuprates: Spin-charge gauge theory versus experiments

The SU(2)xU(1) Chern-Simons spin-charge gauge approach developed earlier to describe the transport properties of the cuprate superconductors in the ``pseudogap'' regime, in particular, the metal-insulator crossover of the in-plane resistivity, is generalized to the ``strange metal'' phase at higher temperature/doping. The short-range antiferromagnetic order and the gauge field fluctuations, which were the key ingredients in the theory for the pseudogap phase, also play an important role in the present case. The main difference between these two phases is caused by the existence of an underlying statistical $\pi$-flux lattice for charge carriers in the former case, whereas the background flux is absent in the latter case. The Fermi surface then changes from small ``arcs'' in the pseudogap to a rather large closed line in the strange metal phase. As a consequence the celebrated linear in T dependence of the in-plane and out-of-plane resistivity is shown explicitly to recover. The doping concentration and temperature dependence of theoretically calculated in-plane and out-of-plane resistivity, spin-relaxation rate and AC conductivity are compared with experimental data, showing good agreement.

In-plane conductivity anisotropy in the underdoped cuprates using the spin-charge gauge approach

Applying the recently developed spin-charge gauge theory for the pseudogap phase in cuprates, we propose a self-consistent explanation of several peculiar features of the far-infrared in-plane AC conductivity, including a broad peak as a function of frequency and significant anisotropy at low temperatures, along with a similar temperature-dependent in-plane anisotropy of DC conductivity in lightly doped cuprates. The anisotropy of the metal-insulator crossover scale is considered to be responsible for these phenomena. The obtained results are in good agreement with experiments. An explicit proposal is made to further check the theory.

Spin-charge gauge approach to “pseudogap”: theory versus experiments

We propose an explanation of several experimental features related to the “pseudogap” in high T c cuprates in terms of a spin-charge gauge theory. In this approach, based on a formal spin-charge separation applied to the t– J model, the low energy effective action describes gapful spinons (with a theoretically derived doping dependence of the gap m s∼ J( δ|ln δ|) 1/2, where δ is the doping concentration) and holons with “small” Fermi surface ( ϵ F∼ tδ) interacting via a gauge field. The main effect of gauge fluctuations is to introduce a dissipation ∼ T/ χ, where χ is the diamagnetic susceptibility. The competition between the two energy scales, m s and T/ χ, is the root in our approach of many phenomena peculiar to transport properties of the “pseudogap phase”. A good agreement is found between the experimental data and theoretically derived doping and temperature dependence of many physical quantities, such as in-plane and out-of-plane resistivity, in-plane magnetoresistance, far infrared electronic AC conductivity and spin lattice relaxation rate.

Spin-charge gauge approach to the pseudogap phase of high-Tccuprates: Theory versus experiments

We consider as a clue to the understanding of the pseudogap phase in high-T-c superconductors the metal-insulator crossover in underdoped, nonsuperconducting cuprates as temperature decreases and a similar crossover in superconducting cuprates when a strong magnetic field suppresses superconductivity. A spin [SU(2)] and charge [U(1)] Chern-Simons gauge field theory, applied to the t-J model, is developed to describe this striking phenomenon. Two length scales have been derived from the theory: the antiferromagnetic correlation length xiapproximate to(deltaparallel toln deltaparallel to)(-1/2), where delta is the doping concentration, and the thermal de Broglie wavelength of the dissipative charge carriers lambda(T)approximate to(Tdelta/t)(-1/2), where T is the temperature, t the hopping integral. At low temperatures xiless than or equal tolambda(T), the antiferromagnetic short-range order dominates, and the charge carriers become localized, showing insulating behavior. On the contrary, at high temperatures xigreater than or similar tolambda(T), the dissipative motion of charge carriers prevails, exhibiting metallic conductivity. Furthermore, the gauge interaction induces binding of spinons (spin excitation) and holons (charge carrier) into electron resonance. This process introduces a new energy scale, the inverse recombination time, which turns out to be essential in the interpretation of the out-of-plane resistivity. The major steps in the theoretical derivation of these results, particularly the calculation of the current-current correlation function and the Green's function for the physical electron, are presented with some detail. The obtained theoretical results are systematically compared with the in-plane and out-of-plane resistivity data and the magnetoresistance data, as well as the nuclear magnetic relaxation data in the pseudogap regime. Very good agreement is obtained.