arXiv subjects

Jian-Xin Zhu

Publications and source records attributed to Jian-Xin Zhu.

At least 181 records · Page 10Linked to original sources

arXiv2012

Superfluid density in the $s_{\pm}$-wave state of clean iron-based superconductors

Based on a phenomenological model and the Kubo formula, we investigate the superfluid density $ρ_s(T)$ and then the penetration depth $λ(T)$ of the iron-based superconductors in the coexistence region of the spin-density-wave(SDW) and superconductivity, and also in the over-doped region. Our calculations show a dramatic increase of $λ(0)$ with the decrease of the doping concentration $x$ below $x=0.1$. This result is consistent with the experimental observations. At low temperatures, $ρ(T)$ shows an exponential-law behavior, while at higher temperatures, the linear-in-$T$ behavior is dominant before it trends to vanish. The evolution of $Δλ(T)$ can be roughly fitted by a exponential function at overdoped levels while in other doping range it is a power-law function with the exponent depending on the doping concentration. We show that the Uemura relation holds for the iron-based superconductors only at very low doping levels.

cond-mat.supr-con↗

arXiv2012

Impurity states in multiband s-wave superconductors: analysis of iron pnictides

We examine the effect of a single, non-magnetic impurity in a multiband, extended s-wave superconductor allowing for anisotropy of the gaps on the Fermi surfaces. We derive analytic expressions for the Green's functions in the continuum and analyse the conditions for the existence of sharp impurity-induced resonant states. Underlying band structure is more relevant for the multiband than for single band case, and mismatch between the bands generically makes the formation of the impurity states less likely in the physical regime of parameters. We confirm these conclusions by numerically solving the impurity problem in a tight-binding parameterization of the bands relevant to pnictide superconductors.

cond-mat.supr-con↗

arXiv2012

Self-consistent spin-fluctuation spectrum and correlated electronic structure of actinides

We present an overview of various theoretical methods with detailed emphasis on an intermediate Coulomb-U coupling model. This model is based on material-specific ab-initio band structure from which correlation effects are computed via self-consistent GW-based self-energy corrections arising from spin-fluctuations. We apply this approach to four isostructural intermetallic actinides PuCoIn5, PuCoGa5, PuRhGa5 belonging to the Pu-115 family, and UCoGa5, a member of the U-115 family. The 115 families share the property of spin-orbit split density of states enabling substantial spin fluctuations around 0.5 eV, whose feedback effect on the electronic structure create mass renormalization and electronic `hot spots`, i.e., regions of large spectral weight. A detailed comparison is provided for the angle-resolved and angle-integrated photoemission spectra and de Haas-van Alphen experimental data as available. The results suggest that this class of actinides is adequately described by the intermediate Coulomb interaction regime, where both itinerant and incoherent features coexist in the electronic structure.

cond-mat.str-el↗

arXiv2012

Imaging the formation of high-energy dispersion anomalies in the actinide UCoGa$_5$

We use angle-resolved photoemission spectroscopy (ARPES) to image the emergence of substaintial dispersion anomalies in the electronic renormalization of the actinide compound UCoGa$_5$ which was presumed to belong to a conventional Fermi liquid family. Kinks or abrupt breaks in the slope of the quasiparticle dispersion are detected both at low ($\sim$130 meV) and high ($\sim$1 eV) binding energies below the Fermi energy, ruling out any significant contribution of phonons. We perform numerical calculations to demonstrate that the anomalies are adequately described by coupling between itinerant fermions and spin fluctuations arising from the particle-hole continuum of the spin-orbit split $5f$ states of uranium. These anomalies are resemble the `waterfall' phenomenon of the high-temperature copper-oxide superconductors, suggesting that spin fluctuations are a generic route toward multiform electronic phases in correlated materials as different as high-temperature superconductors and actinides.

cond-mat.str-el↗

arXiv2012

Pairing Symmetry in Iron-Pnictide Superconductor KFe$_2$As$_2$

The pairing symmetry is one of the major issues in the study of iron-based superconductors. We adopt a low-energy effective kinetic model based on the first-principles band structure calculations combined with the $J_1$-$J_2$ model for KFe$_2$As$_2$, the phase diagram of pairing symmetries is constructed. Putting the values of $J_1$ and $J_2$ of the $J_1$-$J_2$ model obtained by the first-principles calculations into this phase diagram, we find that the pairing symmetry for KFe$_2$As$_2$ is a nodal $d_{xy}$-wave in the folded Brillouin zone with two iron atoms per unit cell. This is in good agreement with experiments observed a nodal order parameter.

cond-mat.supr-con↗

arXiv2012

Temperature-dependent terahertz conductivity of topological insulator Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$

Using Terahertz Time-Domain Spectroscopy, we study the temperature-dependent complex optical conductivity of the topological insulator, Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$ single-crystal from 5 K to 150 K in the terahertz regime (0.4 -- 3.0 THz). We analyze our experimental results using the Drude-Lorentz model, with the Drude component representing the metallic surface state and the Lorentz term representing the bulk insulating state. We find the conductivity to be dominated by the Drude contribution, suggesting the presence of metallic surface states. The low-frequency real conductivity follows a thermally-activated behavior. Its origin is also discussed.

cond-mat.mes-hall↗

arXiv2012

Impurity Quantum Phase Transition in a Current-Carrying d-Wave Superconductor

We study an Anderson impurity embedded in a d-wave superconductor carrying a supercurrent. The low-energy impurity behavior is investigated by using the numerical renormalization group method developed for arbitrary electronic bath spectra. The results explicitly show that the local impurity state is completely screened upon the non-zero current intensity. The impurity quantum criticality is in accordance with the well-known Kosterlitz-Thouless transition.

cond-mat.str-el↗

arXiv2012

Thermoelectric transport with electron-phonon coupling and electron-electron interaction in molecular junctions

Within the framework of nonequilibrium Green's functions, we investigate the thermoelectric transport in a single molecular junction with electron-phonon and electron-electron interactions. By transforming into a displaced phonon basis, we are able to deal with these interactions non-perturbatively. Then, by invoking the weak tunneling limit, we are able to calculate the thermoelectricity. Results show that at low temperatures, resonances of the thermoelectric figure of merit ZT occur around the sides of resonances of electronic conductance but drops dramatically to zero at exactly these resonant points. We find ZT can be enhanced by increasing electron-phonon coupling and Coulomb repulsion, and an optimal enhancement is obtained when these two interactions are competing. Our results indicate a great potential for single-molecular-junctions as good thermoelectric devices over a wide range of temperatures.

cond-mat.mes-hall↗

arXiv2012

Fermi surface evolution and checker-board block-spin antiferromagnetism in $A_x$Fe$_{2-y}$Se$_{2}$

We develop an effective multiorbital mean-field t-J Hamiltonian with realistic tight-binding and exchange parameters to describe the electronic and magnetic structures of iron-selenide based superconductors $A_x$Fe$_{2-y}$Se$_2$ for iron vacancy doping in the range $0 \leq y \leq 0.4$. The Fermi surface topology extracted from the spectral function of angle-resolved photoemission spectroscopy (ARPES) experiments is adequately accounted for by a tight-binding lattice model with random vacancy disorder. Since introducing iron vacancies breaks the lattice periodicity of the stochiometric compound, it greatly affects the electronic band structure. With changing vacancy concentration the electronic band structure evolves, leading to a reconstruction of the Fermi surface topology. For intermediate doping levels, the realized stable electronic structure is a compromise between the solutions for the perfect lattice with $y=0$ and the vacancy stripe-ordered lattice with $y=0.4$, which results in a competition between vacancy random disorder and vacancy stripe order. A multiorbital hopping model is parameterized by fitting Fermi surface topologies to ARPES experiments, from which we construct a mean-field t-J lattice model to study the paramagnetic and antiferromagnetic (AFM) phases of K$_{0.8}$Fe$_{1.6}$Se$_2$. In the AFM phase the calculated spin magnetization of the t-J model leads to a checker-board block-spin structure in good agreement with neutron scattering experiments and {\it ab}-{\it initio} calculations.

cond-mat.supr-con↗

arXiv2012

Doping dependence of the electron-phonon and electron-spin fluctuation interactions in the high-$T_{c}$ superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$

Using ultrafast optical techniques, we detect two types of bosons strongly coupled to electrons in the family of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ from the underdoped to overdoped regimes. The different doping dependences of the electron-boson coupling strengths enables us to identify them as phonons and spin fluctuations: electron-phonon coupling ($λ_{e-ph}$) peaks at optimal doping, and electron-spin fluctuation coupling ($λ_{e-sf}$) decreases monotonically with doping. This observation is consistent with two facts: (1) superconductivity is in close proximity with antiferromagnetism at low dopings, and (2) a pronounced lattice renormalization effect at larger dopings.

cond-mat.supr-con↗

arXiv2012

Local Electronic Structure and Fano Interference in Tunneling into a Kondo Hole System

Motivated by recent success of local electron tunneling into heavy fermion materials, we study the local electronic structure around a single Kondo hole in an Anderson lattice model and the Fano interference pattern relevant to STM experiments. Within the Gutzwiller method, we find that an intragap bound state exists in the heavy Fermi liquid regime. The energy position of the intragap bound state is dependent on the on-site potential scattering strength in the conduction and $f$-orbital channels. Within the same method, we derive a new $dI/dV$ formulation, which includes explicitly the renormalization effect due to the $f$-electron correlation. It is found that the Fano interference gives asymmetric coherent peaks separated by the hybridization gap. The intragap peak structure has a Lorenzian shape, and the corresponding $dI/dV$ intensity depends on the energy location of the bound state.

cond-mat.str-el↗

arXiv2012

Theory of ultrafast quasiparticle dynamics in high-temperature superconductors: Pump fluence dependence

We present a theory for the time-resolved optical spectroscopy of high-temperature superconductors at high excitation densities with strongly anisotropic electron-phonon coupling. A signature of the strong coupling between the out-of-plane, out-of-phase O buckling mode ($B_{1g}$) and electronic states near the antinode is observed as a higher-energy peak in the time-resolved optical conductivity and Raman spectra, while no evidence of the strong coupling between the in-plane Cu-O breathing mode and nodal electronic states is observed. More interestingly, it is observed that under appropriate conditions of pump fluence, this signature exhibits a re-entrant behavior with time delay, following the fate of the superconducting condensate.

cond-mat.supr-con↗

arXiv2012

Superconducting gap structure of the 115's revisited

Density functional theory calculations of the electronic structure of Ce- and Pu-based heavy fermion superconductors in the so-called 115 family are performed. The gap equation is used to consider which superconducting order parameters are most favorable assuming a pairing interaction that is peaked at (π,π,q_z) - the wavevector for the antiferromagnetic ordering found in close proximity. In addition to the commonly accepted $d_{x^2-y^2}$ order parameter, there is evidence that an extended s-wave order parameter with nodes is also plausible. We discuss whether these results are consistent with current observations and possible measurements that could help distinguish between these scenarios.

cond-mat.supr-con↗

arXiv2011

Spin-fluctuations and the peak-dip-hump feature in the photoemission spectrum of actinides

We present first-principles multiband spin susceptibility calculations within the random-phase approximation for four isostructural superconducting PuCoIn$_5$, PuCoGa$_5$, PuRhGa$_5$, and nonsuperconducting UCoGa$_5$ actinides. The results show that a strong peak in the spin-fluctuation dressed self-energy is present around 0.5 eV in all materials, which is mostly created by 5$f$ electrons. These fluctuations couple to the single-particle spectrum and give rise to a peak-dip-hump feature, characteristic of the coexistence of itinerant and localized electronic states. Results are in quantitative agreement with photoemission spectra. Finally, we show that the studied actinides can be understood within the rigid-band filling approach, in which the spin-fluctuation coupling constant follows the same materials dependence as the superconducting transition temperature $T_c$.

cond-mat.supr-con↗

arXiv2011

Local Electronic Structure around a Single Impurity as a Test of Pairing Symmetry in (K,Tl)Fe$_{x}$Se$_{\rm 2}$ Superconductors

We have studied the effect of a single nonmagnetic impurity in the recently discovered (K,Tl)Fe$_x$Se$_2$ superconductors, within both a toy two-band model and a more realistic five-band model. We have found that, out of five types of pairing symmetry under consideration, only the $d_{x^2-y^2}$-wave pairing gives rise to impurity resonance states. The intra-gap states have energies far away from the Fermi energy. The existence of these intra-gap states is robust against the presence or absence of inter-band scattering. However, the inter-band scattering does tune the relative distribution of local density of states at the resonance states. All these features can readily be accessed by STM experiments, and are proposed as a means to test pairing symmetry of the new superconductors.

cond-mat.supr-con↗

arXiv2011

Quasiparticle Scattering Interference in (K,Tl)FexSe2 Superconductors

We model the quasiparticle interference (QPI) pattern in the recently discovered (K,Tl)Fe_xSe2 superconductors. We show in the superconducting state that, due to the absence of hole pockets at the Brillouin zone center, the quasiparticle scattering occurs around the momentum transfer q=(0,0) and (\pm π, \pm π) between electron pockets located at the zone boundary. More importantly, although both d_{x^2-y^2}-wave and s-wave pairing symmetry lead to nodeless quasiparticle excitations, distinct QPI features are predicted between both types of pairing symmetry. In the presence of a nonmagnetic impurity scattering, the QPI exhibits strongest scattering with q=(\pm π, \pm π) for the d_{x^2-y^2}-wave pairing symmetry; while the strongest scattering exhibits a ring-like structure centered around both q=(0,0) and (\pm π, \pm π) for the isotropic s-wave pairing symmetry. A unique QPI pattern has also been predicted due to a local pair-potential-type impurity scattering. The significant contrast in the QPI pattern between the d_{x^2-y^2}-wave and the isotropic s-wave pairing symmetry can be used to probe the pairing symmetry within the Fourier-transform STM technique.

cond-mat.supr-con↗

arXiv2011

Local Moment Formation of an Anderson Impurity on Graphene

We study the property of a magnetic impurity on a single-layer graphene within an Anderson impurity model. Due to the vanishing local density of states at the Fermi level in graphene, the impurity spin cannot be effectively screened out. Treating the problem within the Gutzwiller approximation, we found a region in the parameter space of $U$-$E^f$ where the impurity is in the local moment state, which is characterized by a zero effective hybridization between the bath electron and the magnetic impurity. Here $U$ is the onsite Coulomb repulsion of the impurity electrons and $E^f$ the impurity energy level. The competition between $U$ and $E^f$ is also discussed. While larger $U$ reduces double occupation and favors local moment formation, a deeper impurity level prefers double occupation and a nonzero hybridization and thus a Kondo screened state. For a fixed $U$, by continuously lowering the impurity level, the impurity first enters from a Kondo screened state into a local moment state and then departs from this state and re-enters into the Kondo screened state.

cond-mat.str-el↗

arXiv2011

Electronic Structure and Correlation Effects in PuCoIn5 as compared to PuCoGa5

Since their discovery nearly a decade ago, plutonium-based superconductors have attracted considerable interest, which is now heightened by the latest discovery of superconductivity in PuCoIn5. In the framework of density functional theory (DFT) within the generalized gradient approximation (GGA) together with dynamical mean-field theory (DMFT), we present a comparative study of the electronic structure of PuCoIn5 with the related material, PuCoGa5. Overall, a similar GGA-based electronic structure, including the density of states, energy dispersion, and Fermi surface topology, was found for both compounds. The GGA Pu 5f band was narrower in PuCoIn5 than in PuCoGa5, resulting in an effective reduction of Kondo screening in the former system, as also shown by DMFT calculations. This phenomenon is due to the expanded lattice for PuCoIn5.

cond-mat.str-el↗