arXiv subjects

Jian-Xin Zhu

Publications and source records attributed to Jian-Xin Zhu.

At least 235 records · Page 13Linked to original sources

arXiv2006

Local Strong Coupling Pairing in $D$-Wave Superconductor with Inhomogeneous Bosonic Modes

Recent local tunneling data indicate strong nanoscale inhomogeneity of superconducting gap in high temperature superconductors. Strong local nanoscale inhomogeneity in the bosonic scattering mode has also been observed in the same samples. We argue that these two inhomogeneities directly related to each other. To address local boson scattering effects, we develop a local strong coupling model of superconducting pairing in a coarse grained superconducting state. Each patch is characterized by local coupling to the bosonic mode as well as by local mode energy. We find that local gap value on each patch grows with the local strength of electron-boson interaction. At the same time local gap value decreases with the local boson mode energy, an observation consistent with the tunneling experiments. We argue that features in the tunneling spectrum due to boson scattering are consistent with experimentally observed spectra. We also address the $^{16}O$ to $^{18}O$ isotope substitution. Since both coupling constant and boson energy could change upon isotope substitution, we prove that interplay between these two effects can produce results that are very different from conventional BCS model.

cond-mat.supr-con↗

arXiv2005

Magnetic Quantum Phase Transitions in Kondo Lattices

The identification of magnetic quantum critical points in heavy fermion metals has provided an ideal setting for experimentally studying quantum criticality. Motivated by these experiments, considerable theoretical efforts have recently been devoted to reexamine the interplay between Kondo screening and magnetic interactions in Kondo lattice systems. A local quantum critical picture has emerged, in which magnetic interactions suppress Kondo screening precisely at the magnetic quantum critical point (QCP). The Fermi surface undergoes a large reconstruction across the QCP and the coherence scale of the Kondo lattice vanishes at the QCP. The dynamical spin susceptibility exhibits $ω/T$ scaling and non-trivial exponents describe the temperature and frequency dependence of various physical quantities. These properties are to be contrasted with the conventional spin-density-wave (SDW) picture, in which the Kondo screening is not suppressed at the QCP and the Fermi surface evolves smoothly across the phase transition. In this article we discuss recent microscopic studies of Kondo lattices within an extended dynamical mean field theory (EDMFT). We summarize the earlier work based on an analytical $ε$-expansion renormalization group method, and expand on the more recent numerical results. We also discuss the issues that have been raised concerning the magnetic phase diagram. We show that the zero-temperature magnetic transition is second order when double counting of the RKKY interactions is avoided in EDMFT.

cond-mat.str-el↗

arXiv2005

Competing Orderings in an Extended Falicov-Kimball Model

We present a Hartree-Fock study of the Falicov-Kimball model extended by both on-site and non-local hybridization. We examine the interplay between excitonic effects and the charge-density wave (CDW) instability known to exist at zero hybridization. It is found that the CDW state remains stable in the presence of finite hybridization; for on-site hybridization the Coulomb interaction nevertheless strongly enhances the excitonic average above its value in the noninteracting system. In contrast, for non-local hybridization, we observe no such enhancement of the excitonic average or a spontaneous on-site hybridization potential. Instead, we find only a significant suppression of the excitonic correlations in the CDW state. A phenomenological Ginzburg-Landau analysis is also provided to understand the interplay.

cond-mat.str-el↗

arXiv2005

Spin and Spin-Wave Dynamics in Josephson Junctions

We extend the Keldysh formulation to quantum spin systems and derive exact equations of motion. This allows us to explore the dynamics of single spins and of ferromagnets when these are inserted between superconducting leads. Several new effects are reported. Chief amongst these are nutations of single S=1/2 spins in Josephson junctions. These nutations are triggered by the superconducting pairing correlations in the leads. Similarly, we find that on rather universal grounds, magnets display unconventional spin wave dynamics when placed in Josephson junctions. These lead to modifications in the tunneling current.

cond-mat.supr-con↗

arXiv2005

Dopant-Induced Local Pairing Inhomogeneity in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

A new theoretical model is presented to study the nanoscale electronic inhomogeneity in high-$T_c$ cuprates. In this model, we argue that the randomly distributed out-of-plane interstitial oxygen dopants induces locally the off-diagonal (i.e., hopping integral) disorder. This disorder modulates the superexchange interaction resulting from a large-$U$ Hubbard model, which in turns changes the local pairing interaction. The microscopic self-consistent calculations shows that the large gap regions are registered to the locations of dopants. Large gap regions exhibit small and broader coherence peaks. These results are qualitatively consistent with recent STM observations on optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$.

cond-mat.supr-con↗

arXiv2005

Effects of $τ_1$ Scattering on Fourier-Transformed Inelastic Tunneling Spectra in High-$T_c$ Cuprates with Bosonic Modes

We study the $τ_1$-impurity induced $\mathbf{q}$-space pattern of the energy derivative local density of states (LDOS) in a d-wave superconductor. We are motivated in part by the recent scanning tunneling microscopy (STM) observation of strong gap inhomogeneity with weak charge density variation in Bi$_{2}$Sr$_2$CaCu$_2$O$_{8+δ}$ (BSCCO). The hypothesis is that the gap inhomogeneity might be triggered by the disorder in pair potential. We focus on the effects of electron coupling to various bosonic modes, at the mode energy shifted by the d-wave superconducting gap. The pattern due to a highly anisotropic coupling of electrons to the $B_{1g}$ phonon mode is similar to preliminary results from the Fourier transformed inelastic electron tunneling spectroscopy (FT-IETS) STM experiment in BSCCO. We discuss the implications of our results in the context of band renormalization effects seen in the ARPES experiments, and suggest means to further explore the electron-boson coupling in the high-$T_c$ cuprates.

cond-mat.supr-con↗

arXiv2005

Fourier-Transformed Local Density of States and Tunneling into a $D$-Wave Superconductor with Bosonic Modes

We analyze the effects of the electronic coupling to bosonic modes in a d-wave superconductor. The role of the scattering due to boson on the momentum transfer between electronic states in the Brilloine zone is addressed. We consider specific examples of $B_{1g}$ phonon, breathing mode phonon and spin resonance at $(π,π)$. The Fourier spectrum of the energy derivative local density of states (LDOS) is calculated. To properly calibrate the effects of different modes we fix the quasipartilce renormalization at specific momentum points. It is found that the $B_{1g}$ mode with highly anisotropic momentum-dependent coupling matrix element gives rise to well definded features in the Fourier spectrum, at the energy of mode plus gap, with a momentum transfer along the Cu-O bond direction of cuprates. This result is in a striking contrast to the cases of the coupling to other modes and also to the case of no mode coupling. The origin of this difference is explored in detail. A comparison with the recent STM experiments is briefly discussed.

cond-mat.supr-con↗

arXiv2004

Impurity-induced states in conventional and unconventional superconductors

We review recent developments in our understanding of how impurities influence the electronic states in the bulk of superconductors. Our focus is on the quasi-localized states in the vicinity of impurity sites in conventional and unconventional superconductors and our goal is to provide a unified framework for their description. The non-magnetic impurity resonances in unconventional superconductors are directly related to the Yu-Shiba-Rusinov states around magnetic impurities in conventional s-wave systems. We review the physics behind these states, including quantum phase transition between screened and unscreened impurity, and emphasize recent work on d-wave superconductors. The bound states are most spectacularly seen in scanning tunneling spectroscopy measurements on high-$T_c$ cuprates, which we describe in detail. We also discuss very recent progress on the states coupled to impurity sites which have their own dynamics, and impurity resonances in the presence of an order competing with superconductivity. Last part of the review is devoted to influence of local deviations of the impurity concentration from its average value on the density of states in s-wave superconductors. We review how these fluctuations affect the density of states and show that s-wave superconductors are, strictly speaking, gapless in the presence of an arbitrarily small concentration of magnetic impurities.

cond-mat.supr-con↗

arXiv2004

Equation-of-Motion Approach to Dynamical Mean Field Theory

We propose using an equation-of-motion approach as an impurity solver for dynamical mean field theory. As an illustration of this technique, we consider a finite-$U$ Hubbard model defined on the Bethe lattice with infinite connectivity at arbitrary filling. Depending on the filling, the spectra that is obtained exhibits a quasiparticle peak, and lower and upper Hubbard bands as typical features of strongly correlated materials. The results are also compared and in good agreement with exact diagonalization. We also find a different picture of the spectral weight transfer than the iterative perturbation theory.

cond-mat.str-el↗

arXiv2004

Detecting Breather Excitations with Inelastic Tunneling Spectroscopy

We propose inelastic electron tunneling spectroscopy scanning tunneling microscopy (IETS-STM) as a means of exciting and observing intrinsic localized modes (breathers) in a macromolecule. As a demonstration, inelastic tunneling features of the density of states are calculated for a simple nonlinear elastic Morse chain. The general formalism we have developed for the IETS is applicable to other nonlinear extended objects, such as DNA on a substrate.

cond-mat.soft↗

arXiv2004

Spin and Current Variations in Josephson Junctions

We study the dynamics of a single spin embedded in the tunneling barrier between two superconductors. As a consequence of pair correlations in the superconducting state, the spin displays rich and unusual dynamics. To properly describe the time evolution of the spin we derive the effective Keldysh action for the spin. The superconducting correlations lead to an effective spin action, which is non-local in time, leading to unconventional precession. We further illustrate how the current is modulated by this novel spin dynamics.

cond-mat.mes-hall↗

arXiv2004

Novel Spin Dynamics in a Josephson Junction

We address the dynamics of a single spin embedded in the tunneling barrier between two superconductors. As a consequence of pair correlations in the superconducting state, the spin displays a rich and unusual dynamics. To properly describe the time evolution of the spin we find the generalized Wess-Zumino-Witten-Novikov term in the effective action for the spin on the Keldysh contour. The superconducting correlations lead to an effective spin action which is non-local in time leading to unconventional precessions. Our predictions might be directly tested for macroscopic spin clusters.

cond-mat.mes-hall↗

arXiv2003

Effects of a Collective Spin Resonance Mode on the STM Spectra of D-Wave Superconductors

A high-energy spin resonance mode is known to exist in many high-temperature superconductors. Motivated by recent scanning tunneling microscopy (STM) experiments in superconducting Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, we study the effects of this resonance mode on the local density of states (LDOS). The coupling between the electrons in a d-wave superconductor and the resonance mode produces high-energy peaks in the LDOS, which displays a two-unit-cell periodic modulation around a nonmagnetic impurity. This suggests a new means to not only detect the dynamical spin collective mode but also study its coupling to electronic excitations.

cond-mat.supr-con↗

arXiv2003

Continuous quantum phase transition in a Kondo lattice model

We study the magnetic quantum phase transition in an anisotropic Kondo lattice model. The dynamical competition between the RKKY and Kondo interactions is treated using an extended dynamic mean field theory (EDMFT) appropriate for both the antiferromagnetic and paramagnetic phases. A quantum Monte Carlo approach is used, which is able to reach very low temperatures, of the order of 1% of the bare Kondo scale. We find that the finite-temperature magnetic transition, which occurs for sufficiently large RKKY interactions, is first order. The extrapolated zero-temperature magnetic transition, on the other hand, is continuous and locally critical.

cond-mat.str-el↗

arXiv2003

Atomic scale elastic textures coupled to electrons in superconductors

We present an atomic scale theory of lattice distortions using strain related variables and their constraint equations. Our approach connects constrained atomic length scale variations to continuum elasticity and describes elasticity at all length scales. We apply the general approach to a two-dimensional square lattice with a monatomic basis, and find the atomic scale elastic textures around a structural domain wall and a single defect, as exemplary textures. We clarify the microscopic origin of gradient terms, some of which are included phenomenologically in Landau-Ginzburg theory. The obtained elastic textures are used to investigate the effects of elasticity-driven lattice deformation on the nanoscale electronic structure in superconductor by solving the Bogliubov-de Gennes equations with the electronic degrees of freedom coupled to the lattice ones. It is shown that the order parameter is depressed in the regions where the lattice deformation takes place. The calculated local density of states suggests the electronic structure is strongly modulated as a response to the lattice deformation-- the elasticity propagates the electronic response over long distances. In particular, it is possible for the trapping of low-lying quasiparticle states around the defects. These predictions could be directly tested by STM experiments in superconducting materials.

cond-mat.supr-con↗

arXiv2003

Elasticity-Driven Nanoscale Electronic Structure in Superconductors

The effects of long-range anisotropic elastic deformations on electronic structure in superconductors are analyzed within the framework of the Bogoliubov-de Gennes equations. Cases of twin boundaries and isolated defects are considered as illustrations. We find that the superconducting order parameter is depressed in the regions where pronounced lattice deformation occurs. The calculated local density of states suggests that the electronic structure is strongly modulated in response to lattice deformations, and propagates to longer distances. In particular, this allows the trapping of low-lying quasiparticle states around defects. Some of our predictions can be directly tested by STM experiments.

cond-mat.supr-con↗

arXiv2003

Vortex Core Excitations in Superconductors with Frustrated Antiferromagnetism

Motivated by recent discovery of cobalt oxide and organic superconductors, we apply an effective model with strong antiferromagnetic and superconducting pairing interaction to a related lattice structure. It is found that the antiferromagnetism is highly frustrated and a broken-time-reversal-symmetry chiral $d+id^{\prime}$-wave pairing state prevails. In the mixed state, we have solved the local electronic structure near the vortex core and found no local induction of antiferromagnetism. This result is in striking contrast to the case of copper oxide superconductors. The calculated local density of states indicates the existence of low-lying quasiparticle bound states inside the vortex core, due to a fully gapped chiral pairing state. The prediction can be directly tested by scanning tunneling microscopy.

cond-mat.supr-con↗

arXiv2003

Vibration Mode Induced Shapiro Steps and Backaction in Josephson Junctions

A model of a superconducting tunnel junction coupled to a mechanical oscillator is studied at zero temperature in the case of linear coupling between the oscillator and tunneling electrons. It is found that the Josephson current flowing between two superconductors is modulated by the motion of the oscillator. Coupling to harmonic oscillator produces additional Shapiro steps in $I-V$ characteristic of Josephson junction whose position is tuned by the frequency of the vibration mode. We also find a new velocity dependent term originating from the backaction of the ac Josephson current. This term is periodic in time and vanishes at zero bias voltage.

cond-mat.mes-hall↗