Interband Hopping Research Articles

Reentrant superconductivity due to inter-band off-diagonal hopping in a two-band superconductivity model

Reentrant superconductivity due to inter-band off-diagonal hopping in a two-band superconductivity model

Spin-Triplet Pairing Induced by Spin-Singlet Interactions in Noncentrosymmetric Superconductors

In noncentrosymmetric superconductors, we examine the effect of the difference between the intraband and interband interactions, which becomes more important when the band splitting increases. We define the difference ΔVμ between their coupling constants, i.e., that between the intraband and interband hopping energies of intraband Cooper pairs. Here, the subscript μ of ΔVμ indicates that the interactions scatter the spin-singlet and spin-triplet pairs when μ = 0 and μ = 1,2,3, respectively. It is shown that the strong antisymmetric spin–orbit interaction reverses the target spin parity of the interaction: it converts the spin-singlet and spin-triplet interactions represented by ΔV0 and ΔVμ>0 into effective spin-triplet and spin-singlet pairing interactions, respectively. Hence, for example, triplet pairing can be induced solely by the singlet interaction ΔV0. We name the pairing symmetry of the system after that of the intraband Cooper pair wave function, but with an odd-parity phase factor excluded. The ...

Study of multiband disordered systems using the typical medium dynamical cluster approximation

We generalize the typical medium dynamical cluster approximation to multiband disordered systems. Using our extended formalism, we perform a systematic study of the non-local correlation effects induced by disorder on the density of states and the mobility edge of the three-dimensional two-band Anderson model. We include inter-band and intra-band hopping and an intra-band disorder potential. Our results are consistent with the ones obtained by the transfer matrix and the kernel polynomial methods. We apply the method to K$_x$Fe$_{2-y}$Se$_2$ with Fe vacancies. Despite the strong vacancy disorder and anisotropy, we find the material is not an Anderson insulator. Our results demonstrate the application of the typical medium dynamical cluster approximation method to study Anderson localization in real materials.

Spin-triplet paired state induced by Hund’s rule coupling and correlations: a fully statistically consistent Gutzwiller approach

The intrasite and intersite spin-triplet pairing gaps induced by interband Hund’s rule coupling and their correlations are analyzed in the doubly degenerate Hubbard Hamiltonian. To include the effect of correlations, the statistically consistent Gutzwiller approximation is used. In this approach the consistency means that the averages calculated from the self-consistent equations and those determined variationally coincide with each other. Emphasis is put on the solution for which the average particle number is conserved when carrying out the Gutzwiller projection. This method leads to a stable equal-spin paired state in the so-called repulsive interactions limit (U > 3J) in the regime of moderate correlations. The interband hybridization introduces an inequivalence of the bands which, above a critical magnitude, suppresses the paired state due to both the Fermi-wavevector mismatch for the Cooper pair and the interband hopping allowed by the Pauli principle.

Multi-orbit tight binding calculations for spin transfer torque in magnetic tunneling junctions

We investigate the spin transfer torque (STT) with multi-orbit tight binding model in the magnetic tunneling junctions (MTJs). So far, most of the theoretical works based on the non-equilibrium Keldysh Green’s function method employ a single band model for the simplicity, except a few first principle studies. Even though the single band model captures main physics of STT in MTJ, multi-band calculation reveals new features of the STT that depend on band parameters, such as insulator bandgap, inter-band hopping energy of the ferromagnetic layer. We find that the sign change of perpendicular torkance with bandgap of the insulator layer, and when we allow the inter-band hopping, the bias dependences of perpendicular STT are dramatically changed, while no noticeable changes in parallel STT are found.

A monolithic RF transceiver for DC-OFDM UWB

This paper presents a first monolithic RF transceiver for DC-OFDM UWB applications. The proposed direct-conversion transceiver integrates all the building blocks including two receiver (Rx) cores, two transmitter (Tx) cores and a dual-carrier frequency synthesizer (DC-FS) as well as a 3-wire serial peripheral interface (SPI) to set the operating status of the transceiver. The ESD-protected chip is fabricated by a TSMC 0.13-μm RF CMOS process with a die size of 4.5 × 3.6 mm2. The measurement results show that the wideband Rx achieves an NF of 5–6.2 dB, a max gain of 76–84 dB with 64-dB variable gain, an in-/out-of-band IIP3 of −6/+4 dBm and an input loss S11 of < −10 in all bands. The Tx achieves an LOLRR/IMGRR of −34/-33 dBc, a typical OIP3 of +6 dBm and a maximum output power of −5 dBm. The DC-FS outputs two separate carriers simultaneously with an inter-band hopping time of < 1.2 ns. The full chip consumes a maximum current of 420 mA under a 1.2-V supply.

Dual spin filter effect in a zigzag graphene nanoribbon

By first principle calculations, a dual spin filter effect under finite bias voltages is demonstrated in an antiferromagnetic junction of symmetric zigzag graphene nanoribbon (ZGNR). Unlike conventional spin filter devices using half metallic materials, the up- and down-spin electrons are unidirectionally filtered in the counter direction of the bias voltage, making the junction a dual spin filter. On the contrary, asymmetric ZGNRs do not exhibit such a spin filter effect. By analyzing Wannier functions and a tight-binding model, we clarify that an interplay between the spin polarized band structure of $\pi$ and $\pi^*$ states near the Fermi level and decoupling of the interband hopping of the two states, arising from the symmetry of the wave functions, plays a crucial role in the effect.

Functional renormalization-group study of the doping dependence of pairing symmetry in the iron pnictide superconductors

We use the functional renormalization group to analyze the phase diagram of a four-band model for the iron pnictides subject to band interactions with certain ${A}_{1g}$ momentum dependence. We determine the parameter regimes where an extended $s$-wave pairing instability with and without nodes emerges. For electron doping, the parameter regime in which a nodal gap appears is in correspondence to recent predictions [A. Chubukov et al., arXiv:0903.5547 (unpublished)], however, at very low ${T}_{c}$. Upon hole doping, the $s$-wave gap never becomes nodal: above a critical strength of the intraband repulsion, the system favors an exotic extended $d$-wave instability on the enlarged hole pockets. At half filling, we find that a strong momentum dependence of interband pair hopping yields an extended $s$-wave instability instead of spin-density wave ordering. These results demonstrate that an interaction anisotropy around the Fermi surfaces generally leads to a pronounced sensitivity of the pairing state on the system parameters.

Intrinsic electron and hole bands in electron-doped cuprate superconductors

Based on the analysis of the measurement data of angle-resolved photoemission spectroscopy (ARPES) and optics, we show that the charge transfer gap is significantly smaller than the optical one and is reduced by doping in electron doped cuprate superconductors. This leads to a strong charge fluctuation between the Zhang-Rice singlet and the upper Hubbard bands. The basic model for describing this system is a hybridized two-band $t$-$J$ model. In the symmetric limit where the corresponding intra- and inter-band hopping integrals are equal to each other, this two-band model is equivalent to the Hubbard model with an antiferromagnetic exchange interaction (i.e. the $t$-$U$-$J$ model). The mean-field result of the $t$-$U$-$J$ model gives a good account for the doping evolution of the Fermi surface and the staggered magnetization.

Monotonicd-wave superconducting gap of the optimally dopedBi2Sr1.6La0.4CuO6superconductor by laser-based angle-resolved photoemission spectroscopy

We propose that the upper Hubbard band (electronlike) and the Zhang-Rice singlet band (holelike) are two essential components in describing low-energy excitations of electron-doped cuprate superconductors. We find that the gap between these two bands is significantly smaller than the charge-transfer gap measured by optics and is further reduced upon doping. This indicates that the charge fluctuation is strong and the system is in the intermediate correlation regime. A two-band model is derived. In the limit that the intraband and interband hopping integrals are equal to each other, this model is equivalent to the unconstrained t-J model with on-site Coulomb repulsions.

Impurity Effects in Sign-Reversing Fully Gapped Superconductors: Analysis of FeAs Superconductors

To understand the impurity effect on Tc in FeAs superconductors, we analyze a simple two-band BCS model with repulsive interband interaction. The realized fully-gapped superconducting state with sign reversal, which is predicted by spin fluctuation theories in this compound, is suppressed by impurities due to the interband hopping of Cooper pairs, if the interband impurity scattering $I'$ is equal to the intraband one $I$. When $|I'/I| < 1$, in highly contrast, Tc is almost unchanged by strong impurity scattering since the interband scattering is almost prohibited by the multiple scattering effect. Since $|I'/I| \sim 0.5$ is expected, the robustness of superconductivity against impurities in FeAs superconductors is naturally understood in term of the sign reversing fully-gapped state.

Two-time Green’s functions in superconductivity theory

Based on the method of the equations of motion for two-time Green’s functions, we derive superconductivity equations for different types of interactions related to the scattering of electrons on phonons and spin fluctuations or caused by strong Coulomb correlations in the Hubbard model. We derive an exact Dyson equation for the matrix Green’s function with the self-energy operator in the form of the multiparticle Green’s function. Calculating the self-energy operator in the approximation of noncrossing diagrams leads to a closed system of equations corresponding to the Migdal-Eliashberg strong-coupling theory. We propose a theory of high-temperature superconductivity due to kinematic interaction in the Hubbard model. We show that two pairing channels occur in systems with a strong Coulomb correlation: one due to the antiferromagnetic exchange in interband hopping and the other due to the coupling to spin and charge fluctuations in hopping within one Hubbard band.

Renormalization of Coulomb interactions in [formula omitted]-wave superconductor [formula omitted

We study the renormalized Coulomb interactions due to retardation effect in Na x CoO 2 . Although the Morel Anderson's pseudo-potential for a 1 g orbital μ a 1 g * is relatively large because the direct Coulomb repulsion U is large, that for interband transition between a 1 g and e g ′ orbitals μ a 1 g , e g ′ * is very small since the renormalization factor for pair hopping J is square of that for U. Therefore, the s-wave superconductivity due to valence-band Suhl–Kondo mechanism will survive against strong Coulomb interactions. The interband hopping of Cooper pairs due to shear phonons is essential to understand the superconductivity in Na x CoO 2 .

Antiferromagnetic exchange and spin-fluctuation pairing in cuprate superconductors

Abstract A microscopic theory of superconductivity is formulated within an effective p - d Hubbard model for a CuO 2 plane. By applying the Mori-type projection technique, the Dyson equation is derived for the Green functions in terms of Hubbard operators. The antiferromagnetic exchange caused by interband hopping results in pairing of all carries in the conduction subband and high T c proportional to the Fermi energy. Kinematic interaction in intraband hopping is responsible for the conventional spin-fluctuation pairing. Numerical solution of the gap equation proves the d -wave gap symmetry and defines T c doping dependence. Oxygen isotope shift and pressure dependence of T c are also discussed.

Exchange and spin-fluctuation mechanisms of superconductivity in cuprates

We propose a microscopical theory of superconductivity in CuO$_2$ layer within the effective two-band Hubbard model in the strong correlation limit. By applying a projection technique for the matrix Green function in terms of the Hubbard operators, the Dyson equation is derived. It is proved that in the mean-field approximation d-wave superconducting pairing mediated by the conventional exchange interaction occurs. Allowing for the self-energy corrections due to kinematic interaction, a spin-fluctuation d-wave pairing is also obtained. $T\sb{c}$ dependence on the hole concentration and $\bf k$-dependence of the gap function are derived. The results show that the exchange interaction (which stems from the interband hopping) prevails over the kinematic interaction (which stems from the intraband hopping).

T c enhancement in multiphonon-mediated multiband superconductivity

Abstract We study a model of a multiband Fermi-surface structure to investigate its effect on the superconducting transition temperature in the limit of a high number of bands. We consider a simple limit consisting of an infinite number of identical locally pairwise coupled bands with intraband and interband hopping and a multiband generalized Bardeen-Cooper-Schrieffer Hamiltonian. The self-consistent mean-field system of equations which determines the intraband and interband order parameters decouples to two independent equations, unless the interband hopping integral is non-zero, in which case an energetically stable superconducting phase appears, where both the intraband and the interband gaps are non-zero. We demonstrate that for all values of the interband coupling constant the critical transition temperature is enhanced compared with the pure intraband critical transition temperature. The model is equivalent to a multiple momentum exchange originating from the interband coupling and thus modelling a...

Electronic pairing due to interband and intraband hopping dependence on oxygen covalency in YBaCuO superconductors

We consider the electronic extended model for the superconducting ceramics. In this model the broad electron band formed in the Cu–O 2 plane interacts with the narrow level created by vacancies on the Cu–O chains. It has been assumed that both; the interband hopping integral and the intraband hopping integral depend on the oxygen occupancy. We derive the pairing potential using the diagonalization method for the two band system and subsequently we look for the pairing terms in the remaining interactions, which are; the Coulomb on site repulsion in the broad band and the occupational difference in the intraband and interband hopping interaction. In the result we obtain the pairing potential with reduced Coulomb repulsion and with negative terms, which are proportional to the differences in the oxygen hopping integrals. The effective pairing potential is analysed for the possibility of superconducting transition. It has been shown that the high temperature superconductivity is possible at the reduced values of the Coulomb Δ t=( ii|1/ r| ij) quasi hopping term. The reduction is due to the occupational difference, Δ V, in the interband hopping interaction between O 2− and O 1− oxygen ions and the narrow level. The Coulomb repulsion on the narrow level also reduces required Δ t but at the much smaller amount.

Variational Monte Carlo evaluations of Gutzwiller states for the Anderson-lattice model.

The electronic ground states for two- and three-dimensional Anderson-lattice clusters with large Hubbard repulsion are determined variationally within the Gutzwiller ansatz by use of a stochastic algorithm for exact evaluation of matrix elements of Gutzwiller states. The algorithm is similar to one normally used for finite-temperature simulation and is described in some detail. Variational states for a variety of Hamiltonian parameters and particle densities were considered. Of particular interest is the regime of small interband hopping, which is antiferromagnetic at densities near two particles per unit cell. We found that for some Hamiltonian-parameter regimes, doping the antiferromagnetic cluster with enough particles or holes can destroy the magnetic ordering and produce a state that resembles a heavy-Fermi-liquid state, a metal with a strongly-frequency-dependent self-energy. For a 4\ifmmode\times\else\texttimes\fi{}4 Anderson lattice, we present results of low-temperature quantum Monte Carlo simulations to evaluate the validity of the variational Gutzwiller ansatz.