arXiv subjects

Jian-Xin Li

Publications and source records attributed to Jian-Xin Li.

89 records · Page 5Linked to original sources

arXiv2009

Band renormalization and Fermi surface reconstruction in iron-based superconductors

Using the fluctuation exchange approximation and a three-orbital model, we study the band renormalization, Fermi surface reconstruction and the superconducting pairing symmetry in the newly-discovered iron-based superconductors. We find that the inter-orbital spin fluctuations lead to the strong anisotropic band renormalization and the renormalization is orbital dependent. As a result, the topology of Fermi surface displays distinct variation with doping from the electron type to the hole type, which is consistent with the recent experiments. This shows that the Coulomb interactions will have a strong effect on the band renormalization and the topology of the electron Fermi pocket. In addition, the pairing state mediated by the inter-orbital spin fluctuation is of an extended s-wave symmetry.

cond-mat.supr-con↗

arXiv2008

Spin Fluctuations, Interband Coupling, and Unconventional Pairing in Iron-based Superconductors

Based on an effective two-band model and using the fluctuation-exchange (FLEX) approach, we explore spin fluctuations and unconventional superconducting pairing in Fe-based layer superconductors. It is elaborated that one type of interband antiferromagnetic (AF) spin fluctuation stems from the interband Coulomb repulsion, while the other type of intraband AF spin fluctuation originates from the intraband Coulomb repulsion. Due to the Fermi-surface topology, a spin-singlet extended s-wave superconducting state is more favorable than the nodal $d_{XY}$-wave state if the interband AF spin fluctuation is more significant than the intraband one, otherwise vice versa. It is also revealed that the effective interband coupling plays an important role in the intraband pairings, which is a distinct feature of the present two-band system.

cond-mat.supr-con↗

arXiv2008

Band structure renormalization and weak pseudogap behavior in Na_{0.33}CoO_2: Fluctuation exchange study based on a single band model

Based on a single band Hubbard model and the fluctuation exchange approximation, the effective mass and the energy band renormalization in Na$_{0.33}$CoO$_2$ is elaborated. The renormalization is observed to exhibit certain kind of anisotropy, which agrees qualitatively with the angle-resolved photoemission spectroscopy (ARPES) measurements. Moreover, the spectral function and density of states (DOS) in the normal state are calculated, with a weak pseudogap behavior being seen, which is explained as a result of the strong Coulomb correlations. Our results suggest that the large Fermi surface (FS) associated with the $a_{1g}$ band plays likely a central role in the charge dynamics.

cond-mat.str-el↗

arXiv2007

Phase Transition in a Two-level-cavity System in an Ohmic Environment

We propose that in the presence of an Ohmic, de-phasing type environment, a two-level-cavity system undergoes a quantum phase transition from a state with damped Rabi oscillation to a state without. We present the phase diagram and make predictions for pump and probe experiment. Such a strong coupling effect of the environment is beyond the reach of conventional perturbative treatment.

cond-mat.mtrl-sci↗

arXiv2006

Photoisomerization in a Dissipative Environment

We investigate photoisomerization (PI), the shape change of a molecule upon photoabsorption, in a dissipative environment using a simple spin-boson model. We identify two classes of environment depending on whether it "entangles" with the molecule. In the absence of entanglement, the environment merely causes a blue shift of the required photon frequency and reduces the quantum efficiency of PI. With entanglement the molecule can undergo a quantum phase transition between a state that photoisomerizes to a state that does not.

cond-mat.mtrl-sci↗

arXiv2006

Doping dependance of the spin resonance peak in bilayer high-$T_c$ superconductors

Motivated by a recent experiment on the bilayer Y$_{1-x}$Ca$_{x}$Ba$_2$Cu$_3$O$_y$ superconductor and based on a bilayer $t-J$ model, we calculate the spin susceptibility at different doping densities in the even and odd channels in a bilayer system. It is found that the intensity of the resonance peak in the even channel is much weaker than that in the odd one, with the resonance position being at a higher frequency. While this difference decreases as the doping increases, and both the position and amplitude of the resonance peaks in the two channels are very similar in the deeply overdoped sample. Moreover, the resonance frequency in the odd channel is found to be linear with the critical temperature $T_c$, while the resonance frequency increases as doping decreases in the even channel and tends to saturate at the underdoped sample. We elaborate the results based on the Fermi surface topology and the d-wave superconductivity.

cond-mat.supr-con↗

arXiv2006

Checkerboard charge density wave and pseudogap in high-$T_{c}$ cuprates

We consider the scenario where a 4-lattice constant, rotationally symmetric charge density wave (CDW) is present in the underdoped cuprates. We prove a theorem that puts strong constraint on the possible form factor of such a CDW. We demonstrate, within mean-field theory, that a particular form factor within the allowed class describes the angle-resolved photoemission and scan tunneling spectroscopy well. We conjecture that the ``large pseudogap'' in cuprates is the consequence of this type of charge density wave.

cond-mat.supr-con↗

arXiv2006

Theory of optical conductivity in detwinned YBa$_2$Cu$_3$O$_{y}$

We study theoretically the chain and in-plane optical conductivities of detwinned YBa$_2$Cu$_3$O$_{y}$. We elaborate that the chain is superconducting for $y>6.67$ while insulating for $y<6.67$ due to the competition between the plane-chain coupling and the antiferromagnetic order, corresponding to a superconductor-insulator transition. Stemming also from the coupling between the plane and chain, a new peak emerges at a low frequency in the in-plane spectra in the superconducting state, while it disappears in the normal state. Our scenario accounts satisfactorily for very recent experiments.

cond-mat.supr-con↗

arXiv2005

Anisotropy of incommensurate spin and charge fluctuations in detwinned YBa$_2$Cu$_3$O$_{6+δ}$

Motivated by a recent neutron scattering experiment on detwinned YBa$_2$Cu$_3$O$_{6+δ}$ superconductor, we examine the frequency and doping dependence of the anisotropy in the spin and charge fluctuation arising from the coupling between the plane and the chain. Starting from the two-dimensional $t$-$t^{'}$-$J$ model and using the random-phase approximation (RPA), we find a pronounced anisotropy of the incommensurate (IC) peaks in the spin channel, namely the peak intensity at the $(π\pmδ,π)$ direction is stronger than that at the $(π,π\pmδ)$ direction in a wide frequency range from $ω=0.2J$ to the resonance frequency $ω_r=0.35J$. Above the resonance frequency, the IC peaks reemerge. Their intensities shift to the diagonal direction and no anisotropy exists along the two diagonal directions. We find that this anisotropy is robust with respect to the possible variation of the RPA correction factor and to the dopings. The charge fluctuation is also found to be incommensurate for all energies considered and peak at $(0,δ)$ and $(δ,0)$. An anisotropy in its IC peak intensity along the $k_x$ and the $k_y$ direction exists, but in sharp contrast to the spin channel, the maximum intensity of the IC peak is along the $k_y$ direction. Moreover, the IC peak in the charge channel exhibits an upward dispersion, in contrast to the downward dispersion below the spin resonance frequency for the spin IC peak. We explain these features based on the effect of the plane-chain coupling on the topology of the Fermi surface.

cond-mat.supr-con↗

arXiv2005

Theoretical understanding of the quasiparticle dispersion in bilayer high-$T_{c}$ superconductors

The renormalization of quasiparticle (QP) dispersion in bilayer high-$T_{c}$ cuprates is investigated theoretically by examining respectively the interactions of the QP with spin fluctuations (SF) and phonons. It is illustrated that both interactions are able to give rise to a kink in the dispersion around the antinodes (near $(π,0)$). However, remarkable differences between the two cases are found for the peak/dip/hump structure in the lineshape, the QP weight, and the interlayer coupling effect on the kink, which are suggested to serve as a discriminance to single out the dominant interaction in the superconducting state. A comparison to recent photoemission experiments shows clearly that the coupling to the spin resonance is dominant for the QP around antinodes in bilayer systems.

cond-mat.supr-con↗

arXiv2003

Spin resonance peak in Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O superconductors: A probe of the pairing symmetry

Motivated by the recent discovery of superconductivity in layered Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O compound, we investigate theoretically the spin dynamics in the superconducting state with three possible $p_{x}+ip_{y}$-, $d+id'$-, and $f$-wave pairing symmetries in two-dimensional triangular lattices. We find that a spin resonance peak, which is elaborated to have a close relevance to the relative phase of the gap function and the geometry of the Fermi surface, appears in both in-plane and out-of-plane components of the spin susceptibility $χ$ for the spin-singlet $d+id'$-wave pairing, while only in the out-of-plane(in-plane) component of $χ$ for the spin-triplet $p_{x}+ip_{y}$-wave ($f$-wave) pairing. Because the pairing symmetry in this compound is still hotly debated at present, this distinct feature may be used to probe or determine it unambiguously with future neutron scattering experiments.

cond-mat.supr-con↗

arXiv2002

Test for Pairing Symmetry Based on Spin Fluctuations in Organic Superconductors $κ$-(BEDT-TTF)$_{2}$X

We propose that the superconducting pairing symmetry of organic superconductors $κ$-(BEDT-TTF)$_{2}$X can be determined by measuring the position in momentum space of the incommensurate peaks of the spin susceptibility. We show that the positions of these peaks are uniquely determined by the symmetry of the superconducting state and the geometry of Fermi surface. Based on the effective Hubbard model on an anisotropic triangular lattice, we demonstrate the different incommensurate patters of spin responses for $d_{x^{2}-y^{2}}$- and $d_{xy}$-like pairing states. In addition, we predict the nonexistence of the spin resonance mode in this compound, which is an important feature in high-$T_{c}$ superconductors.

cond-mat.supr-con↗

arXiv2001

Doping dependence of the resonance peak and incommensuration in high-$T_{c}$ superconductors

The doping and frequency evolutions of the incommensurate spin response and the resonance mode are studied based on the scenario of the Fermi surface topology. We use the slave-boson mean-field approach to the $t-t^{\prime}-J$ model and including the antiferromagnetic fluctuation correction in the random-phase approximation. We find that the equality between the incommensurability and the hole concentration is reproduced at low frequencies in the underdoped regime. This equality observed in experiments was explained {\it only} based on the stripe model before. We also obtain the downward dispersion for the spin response and predict its doping dependence for further experimental testing, as well as a proportionality between the low-energy incommensurability and the resonance energy. Our results suggest a common origin for the incommensuration and the resonance peak based on the Fermi surface topology and the d-wave symmetry.

cond-mat.supr-con↗

arXiv2001

Superconducting condensation energy and neutron resonance in high-Tc superconductors

Based on the slave-boson theory of the two-dimensional $t-t'-J$ model, we calculate the superconducting condensation energy for optimally doped and overdoped high $T_{c}$ cuprates at finite temperatures using a renormalized random phase approximation. The contributions come from the mean-field part and the antiferromagnetic spin fluctuations. In the presence of neutron resonance peak at $(π,π)$, the latter is shown to have similar temperature and doping dependences as the difference in antiferromagnetic (AF) exchange energy between the normal and the superconducting state. This difference has been proposed to be related to the superconducting condensation energy by Scalapino and White. The total condensation energy, however, is about 1/2 smaller than the proposed AF exchange energy difference and shows a more rapid decrease as the temperature rises. The doping dependence of the condensation energy is found to be consistent with experiments. In particular, near zero temperature, our result shows a good quantitative agreement with experiments.

cond-mat.supr-con↗

arXiv1999

A Consistent Picture for Resonance Neutron Peak and ARPES Spectra in High-Tc Superconductors

The spectra observed in ARPES measurements are examined together with the resonance peak observed in neutron scatterings, based on the slave-boson approach to $t-t^{\prime}-J$ model. We show that the peak/dip/hump features arise from the scattering of electrons by collective spin excitations which, at the same time, give rise to the neutron resonance mode. The doping dependences and the dispersions of the peak/dip/hump positions are shown to be consistent with experiments. Our results indicate that recently observed $\cos (6θ)$ deviation from the pure d-wave also result from the renormalization by spin fluctuations.

cond-mat.supr-con↗

arXiv1999

Impurity effects on the spin excitation spectra in a d-wave superconductor

The effects of nonmagnetic impurity on the spin excitation spectra in a $d_{x^2-y^2}$-wave superconductor are examined, using the self-consistent $t$-matrix approximation. It is shown that the impurity self-energy acts to shift the position of the resonance peak to low frequencies and broaden the peak. While the impurity vertex correction causes a broad spectral weight in the spin gap at the impurity concentrations where no clear resonance peak is observed. The gaplike feature still remains in low frequency region upon the introduction of impurities. Incorporating these two effects, we find that the result is in qualitative agreement with experiments on YBa_{2}(Cu_{1-x}Zn_{x})_{3}O_{6+y}.

cond-mat.supr-con↗

arXiv1999

Effect of the Fermi surface destruction on transport properties in underdoped cuprates

Motivated by recent experimental measurements on the Fermi surface(FS) destruction in underdoped high-$T_{c}$ cuprates, we examine its effect on the transport properties based on the Boltzmann equation approach. The effect is modeled by simply taking the density of states for electrons in the gapped regions to be zero. Within the nearly antiferromagnetic Fermi liquid model, we calculate the temperature dependences of the dc resistivity, the inverse Hall angle and the Hall coefficient. It is shown that the effect of the FS destruction on transport properties is sensitive to the existance and the range of the flat band near $(0,\pmπ)$ in the dispersion of electrons, and the anistropy of the relaxation rate along the Fermi surface. We find that the experimental data are better described by the cold spot model, i.e., the transports are determined mainly by the contribution of the electrons near the Brillouin-zone diagonals.

cond-mat.supr-con↗