Slave-boson Mean-field Theory Research Articles

Finite-size effect and Kondo screening effect in an A-B ring with a quantum dot

The properties of the ground state of a closed dot–ring system with a magnetic flux in the Kondo regime are studied theoretically by means of a one-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that at T=0, a suppressed Kondo effect exists in this system even when the mean level spacing of electrons in the ring is larger than the bulk Kondo temperature. The physical quantities depend sensitively on both the parity of the system and the size of the ring; the rich physical behaviour can be attributed to the coexistence of both the finite-size effect and the Kondo screening effect. It is also possible to detect the Kondo screening cloud by measuring the persistent current or the zero field impurity susceptibility χimp directly in future experiments.

Landau transport equations in a slave-boson mean-field theory of thet−Jmodel

In this paper we generalize slave-boson mean-field theory for $t-J$ model to the time-dependent regime, and derive transport equations for $t-J$ model, both in the normal and superconducting states. By eliminating the boson and constraint fields exactly in the equations of motion we obtain a set of transport equations for fermions which have the same form as Landau transport equations for normal Fermi liquid and Fermi liquid superconductor, respectively with all Landau parameters explicity given. Our theory can be viewed as a refined version of U(1) Gauge theory where all lattice effects are retained and strong correlation effects are reflected as strong Fermi-liquid interactions in the transport equation. Some experimental consequences are discussed.

Dopedt−Jmodel on a triangular lattice: Possible application toNaxCoO2⋅yH2OandNa1−xTiO2

We report the finding of time-reversal-symmetry-breaking $d_{x^2-y^2}+ id_{xy}$ superconducting ground state in the slave-boson mean-field theory for the t-J model on triangular lattice. For $t/J=-5$ ($t/J=-9$) pairing exists for $x < 13%$ ($x<8%$) upon electron doping, and $x <56%$ ($x<13%$) upon hole doping. These results are potentially relevant to doped Mott insulators Na$_x$CoO$_2$.yH$_2$O and Na$_{1-x}$TiO$_2$.

The persistent current in an Aharonov–Bohm ring with a side-coupled quantum dot

We have investigated the persistent current in a mesoscopic ring with a side-coupled quantum dot. The problems are probed by using the one-impurity Anderson Hamiltonian and are treated with the slave boson mean field theory. It is shown that the persistent current in this system has the spin fluctuations and the charge transfers between the two subsystems are suppressed in the limit of Δ/T K 0<<1. The minimum value of the persistent current for ξK/L = 5 of the odd parity system provides an opportunity to detect the Kondo screening cloud.

The persistent current in a mesoscopic Aharonov-Bohm ring with a series-coupled double quantum dot

We study theoretically the properties of the ground state of a mescopic ring with a seriescoupled double quantum dot in the Kondo regime by means of the two-inpurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that two dots can be coupled coherently, which is reflected in the appearance of parity effects and the complex current-phase relation in this system. This system might be a possile candidate for future device applications.

COEXISTENCE OF FERROMAGNETISM AND SUPERCONDUCTIVITY IN THE TWO-DIMENSIONAL FERROMAGNETIC t–J MODEL

The two-dimensional ferromagnetic t–J model is studied in the framework of slave-boson mean field theory. After solving the mean field self-consistent equations, we find that there exists a hole-doping region δ1&lt;δ&lt;δ2 in which p-wave superconductivity and ferromagnetism coexist. There also exists a T* line, under which pre-paired p-wave cooper pairs appear. The similarity and differences between our theoretical results and the experimental ones made for UGe 2 are compared.

Statistics of spinons in the spin-liquid phase ofCs2CuCl4

Motivated by a recent experiment on ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ [Coldea et al., Phys. Rev. Lett. 86, 1335 (2001)], we study the spin dynamics of the spin-liquid phase of the $\mathrm{s}\mathrm{p}\mathrm{i}\mathrm{n}\ensuremath{-}\frac{1}{2}$ frustrated Heisenberg antiferromagnet on the anisotropic triangular lattice. There have been two different proposals for the spin-liquid phase of ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}.$ These spin-liquid states support different statistics of spinons; the bosonic $\mathrm{Sp}(N)$ large-N mean-field theory predicts bosonic spinons, while the SU(2) slave-boson mean-field theory leads to fermionic spinons. We compute the dynamical spin structure factor for both types of spin-liquid state at zero and finite temperatures. While at zero temperature both theories agree with experiment on a qualitative level, they show substantial differences in the temperature dependence of the dynamical spin structure factor.

Kondo screening cloud effects in mesoscopic devices

We study how finite size effects may appear when a quantum dot in the Kondo Coulomb blockade regime is embedded into a mesoscopic device with finite wires. These finite size effects appear when the size of the mesoscopic device containing the quantum dot is of the order of the size of Kondo cloud and affect all thermodynamic and transport properties of the Kondo quantum dot. We also generalize our results to the experimentally relevant case where the wires contain several transverse modes/channels. Our results are based on perturbation theory, Fermi liquid theory and slave boson mean field theory.

Noise power spectrum of a long-channel current line with electron traps: Slave-boson mean field theory

We calculated the noise power spectrum of a one-dimensional free electron system corresponding to a long-channel current line when there are several electron traps near the current line. The calculation was performed starting from the Anderson Hamiltonian in the framework of the slave-boson mean field theory. When there is a single trap, the noise power spectrum shows a peak structure. When there are two traps and the distance between the two traps is less than the Fermi momentum, the peak of the noise power is enhanced because of interference between the two traps. When there are multiple traps, the noise power is analytically found to have an ω−2 dependence in the low-frequency limit and an ω−1/2 dependence in the high-frequency limit. These results are applicable to the noise analysis of nanodevices such as a single-electron transistor (SET) if the traps are regarded as the SET islands.

Coherent Coupling of Double Quantum Dots Embedded in aMesoscopic Ring

We theoretically study the properties of the ground state of aseries-coupled double quantum dot embedded in a mesoscopic ring in theKondo regime by means of the two-impurity Anderson Hamiltonian. TheHamiltonian is solved by means of the slave-boson mean-field theory. Itis shown that two dots can be coupled coherently, which is reflected inthe appearance of parity effects and the complex current-phase relationin this system. This system might be a possible candidate for futuredevice applications.

Role of Spinon Excitations on KT Transition at Low Energies in d-Wave Superconductors

By using a low energy effective Lagrangian of the U(1) slave boson mean field theory we examine the vortex binding-unbinding transition (KT transition) from a doped Mott insulator at low temperatures in a d-wave superconductor. It is shown that the interaction potential between vortices is modified by a "Doppler energy shift" resulting in additional attractive interaction, as a consequence of supercurrent fluctuations coupled to the particle-hole excitations of the massless Dirac fermions.

Tunneling Spectra of Inhomogeneous d-Wave Superconductor

The tunneling spectrum of an inhomogeneous d-wave superconductor is discussed in the framework of self-consistent slave-boson mean-field theory. Distinct from the usual BCS-type mean-field theory, an electron is now described using both fermionic and bosonic degree of freedom. We show that one can define two types of tunneling spectra in this theory, which also corresponds to two ways of calibrating the STM spectra. In good agreement with the experimental observation, we show that one type of tunneling spectrum remains inhomogeneous while the other type shows a remarkable degree of homogeneity at low energy, despite the underlying disorder. Physical implications of this result, in particular in relation to thermodynamic measurement such as heat capacity, are discussed.

PossibleZ2phase and spin-charge separation in electron-doped cuprate superconductors

The SU(2) slave-boson mean-field theory for the ${\mathrm{tt}}^{\ensuremath{'}}J$ model is analyzed. The role of next-nearest-neighbor hopping ${t}^{\ensuremath{'}}$ on the phase diagram is studied. We find a pseudogap phase in hole-doped (HD) materials (where ${t}^{\ensuremath{'}}&lt;0).$ The pseudogap phase is a U(1) spin liquid (the staggered-flux phase) with a U(1) gauge interaction and no fractionalization. This agrees with experiments on HD samples. The same calculation also indicates that a positive ${t}^{\ensuremath{'}}$ favors a ${Z}_{2}$ state with true spin-charge separation. The ${Z}_{2}$ state that exists when ${t}^{\ensuremath{'}}\ensuremath{\gtrsim}0.5J$ can be a candidate for the pseudogap phase of electron-doped (ED) cuprates (if such a phase exists). The experimental situation in ED materials is also addressed.

Novel boundary effect in high temperature superconductors: superconductivity induced by pseudogap proximity effect

The boundary between highly under-doped and over-doped cuprate superconductors is studied within the framework of the slave-boson mean-field theory of the t – J model. We assume that the under-doped region is in the pseudogap state and the singlet resonating valence bond (s-RVB) order is formed there. The over-doped region, on the other hand, is in a hole-rich state and the pseudogap is destroyed by the holes. Because of the proximity effect, the s-RVB order and high-density holes can coexist, leading to the superconducting behavior. Remarkably, the transition temperature of this “boundary superconductivity” exceeds the maximum superconducting transition temperature of the bulk region, thus possessing a new possibility of application for superconducting devices.

Staggered flux state of electrons in a two-dimensionalt−Jmodel

The competition between the staggered flux state, or the d-density wave state, and the d-wave pairing state is analyzed in a two-dimensional $t\ensuremath{-}J$ model based on the U(1) slave boson mean-field theory. Not only staggered flux of spinons but also staggered flux of holons are considered. In this formalism, the hopping order parameter of physical electron is described by the product of hopping order parameters of spinons and holons. The staggered flux amplitude of electrons is the difference of staggered flux amplitude of spinons and that of holons. In the $\ensuremath{\pi}$-flux phase of spinons, staggered fluxes of spinons and holons cancel completely and the staggered flux order of electrons does not exist. However, in the staggered flux phase of spinons whose staggered flux amplitude is not $\ensuremath{\pi},$ fluxes does not cancel completely and the staggered flux amplitude of electron remains. Thus the phase transition between these two phases, the $\ensuremath{\pi}$-flux phase and the staggered flux phase of spinons, becomes a second-order transition in the physical electron picture. The order parameter which characterizes this transition is the staggered flux order parameter of electrons. A mean-field phase diagram is shown. It is proved analytically that there is no coexistence of staggered flux and d-wave pairing. The temperature dependence of Fermi surface and excitation gap at $(0,\ensuremath{\pi})$ are shown. These behaviors are consistent with angle-resolved photoemission spectroscopy experiments.

Heterogeneous high temperature cuprate superconductors

The boundary between highly under-doped and over-doped cuprate superconductors is studied within the framework of the slave-boson mean-field theory of the t–J model. Because of the proximity effect of singlet resonating valence bond order, the superconductivity appears at the boundary even when there is no bulk superconductivity. Remarkably, the transition temperature of this “boundary superconductivity” exceeds the maximum superconducting transition temperature of the base material in the bulk.

Structure and transport in multi-orbital Kondo systems

We consider Kondo impurity systems with multiple local orbitals, such as rare earth ions in a metallic host or multi-level quantum dots coupled to metallic leads. It is shown that the multiplet structure of the local orbitals leads to multiple Kondo peaks above the Fermi energy E F, each one with its own Kondo temperature T K, and to “shadow” peaks below E F. We use a slave boson mean field theory, which recovers the strong coupling Fermi liquid fixed point, to calculate the Kondo peak positions, widths, and heights analytically at T=0, and NCA calculations to fit the temperature dependence of high-resolution photoemission spectra of Ce compounds. In addition, an approximate conductance quantization for transport through multi-level quantum dots or single-atom transistors in the Kondo regime due to a generalized Friedel sum rule is demonstrated.

Zero-bias conductance peak in tunneling spectroscopy of hybrid superconductor junctions

A generalized method of image, incorporated with the non-equilibrium Keldysh-Green's function formalism, is employed to investigate the tunneling spectroscopy of hybrid systems in the configuration of planar junction. In particular, tunneling spectroscopies of several hybrid systems that exhibit zero-bias conductance peaks (ZBCP) are examined. The well-known metal--d-wave superconductor (ND) junction is first examined in detail. Both the evolution of the ZBCP versus doping and the splitting of the ZBCP in magnetic fields are computed in the framework of the slave-boson mean field theory. Further extension of our method to analyze other states shows that states with particle-hole pairing, such as d-density wave and graphene sheet, are all equivalent to a simple 1D model, which at the same time also describes the polyacetylene. We provide the criteria for the emergence of ZBCP. In particular, broken reflection symmetry at the microscopic level is shown to be a necessary condition for ZBCP to occur.

Size dependence of the persistent currents in an Aharonov–Bohm ring with an in-line quantum dot

The properties of the ground state of a closed dot-ring system with a magnetic flux in the Kondo regime are studied theoretically by means of a one-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that the persistent current in this system induced by a magnetic flux depends sensitively on both the parity of this system and the size of the ring. Some new results are obtained and relevant underlying physics of this problem is discussed.

Pressure effects on the Hall coefficient of Kondo insulators

The effects of pressure on the Hall coefficient of Kondo insulators are studied in the framework of the slave-boson mean-field theory under the coherent potential approximation. A unified picture is presented for both the hole-type Kondo insulators and the electron-type Kondo insulators. The density of states of f-electrons under the applied pressure and its variation with the concentration of the impurity doping are calculated self-consistently. The Hall coefficient for the hole-type Kondo insulators and the Kondo temperature are obtained, in agreement with experiment qualitatively. The Hall coefficient for the electron-type Kondo insulators and the static zero-temperature magnetic susceptibility under the various pressure are obtained and are in qualitative agreement with experiment.