arXiv subjects

Jian-Xin Zhu

Publications and source records attributed to Jian-Xin Zhu.

At least 163 records · Page 9Linked to original sources

arXiv2014

Asymmetric Andreev Reflection Induced Electrical and Thermal Hall-like Effects in Metal/Anisotropic Superconductor Junctions

By investigating the nonequilibrium transport across a metal/superconductor junction in both nonrelativistic and relativistic cases, we reveal that the asymmetric Andreev reflection with anisotropic superconductors is able to induce the electric and thermal Hall-like effect, in the absence of magnetic field. That is, a longitudinal electric voltage or temperature bias can induce transverse electric or thermal currents merely through the asymmetric Andreev reflection, respectively. In particular, a transverse thermoelectric effect, i.e., the Ettingshausen-like effect, is identified, although the conjugate Nernst effect is absent. The direction change of these electric and thermal Hall-like currents is also discussed. The Hall-like effects uncovered here do not require the conventional time-reversal symmetry breaking, but rather originate from the mirror symmetry breaking with respect to the interface normal due to the anisotropic paring symmetry of the superconductor.

cond-mat.mes-hall↗

arXiv2014

Pressure induced topological quantum phase transition in Sb_2Se_3

Based on the first-principles band structure calculations, we investigate the effects of hydrostatic pressure on the conventional insulator (CI) Sb_2Se_3 and predict that it undergoes a topological quantum phase transition from a CI to a nontrivial topological insulator at a critical pressure value. The pressure-induced topological quantum phase transition is confirmed by calculating the evolution of the bulk energy gap as a function of pressure, the inversion of energy band structure, and the Z_2 topological invariant, as well as the existence of the Dirac-like topological surface states. Our predictions can be tested by both spectroscopy and transport experiments.

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arXiv2013

GW quasiparticle calculations with spin-orbit coupling for the light actinides

We report on the importance of GW self-energy corrections for the electronic structure of light actinides in the weak-to-intermediate coupling regime. Our study is based on calculations of the band structure and total density of states of Np, U, and Pu using a one-shot GW approximation that includes spin-orbit coupling within a full potential LAPW framework. We also present RPA screened effective Coulomb interactions for the f-electron orbitals for different lattice constants, and show that there is an increased contribution from electron-electron correlation in these systems for expanded lattices. We find a significant amount of electronic correlation in these highly localized electronic systems.

cond-mat.str-el↗

arXiv2013

Evolution of the Fermi surface topology in doped 122 iron pnictides

Based on the minimum two-orbital model and the phase diagram recently proposed by Tai et al.\ (Europhys. Lett. \textbf{103}, 67001(2013)) for both electron- and hole-doped 122 iron-based superconducting compounds, we use the Bogoliubov-de Gennes equations to perform a comprehensive investigation of the evolution of the Fermi surface (FS) topology in the presence of the collinear spin-density-wave (SDW) order as the doping is changed. In the parent compound, the ground state is the SDW order, where the FS is not completely gapped, and two types of Dirac cones, one electron-doped and the other hole-doped emerge in the magnetic Brillouin zone. Our findings are qualitatively consistent with recent angle-resolved photoemission spectroscopy and magneto-resistivity measurements. We also examine the FS evolution of both electron- and hole-doped cases and compare them with measurements, as well as with those obtained by other model Hamiltonians.

cond-mat.supr-con↗

arXiv2013

Site-Selective Electronic Correlation in alpha-Plutonium Metal

An understanding of the phase diagram of elemental plutonium (Pu) must include both the effects of the strong directional bonding and the high density of states of the Pu 5f electrons, as well as how that bonding weakens under the influence of strong electronic correlations. We present for the first time electronic-structure calculations of the full 16-atom per unit cell alpha-phase structure within the framework of density functional theory (DFT) together with dynamical mean-field theory (DMFT). Our calculations demonstrate that Pu atoms sitting on different sites within the alpha-Pu crystal structure have a strongly varying site dependence of the localization-delocalization correlation effects of their 5f electrons and a corresponding effect on the bonding and electronic properties of this complicated metal. In short, alpha-Pu has the capacity to simultaneously have multiple degrees of electron localization/delocalization of Pu 5f electrons within a pure single-element material.

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arXiv2013

Superconductivity at the Border of Electron Localization and Itinerancy

The superconducting state of iron pnictides and chalcogenides exists at the border of antiferromagnetic order. Consequently, these materials could provide clues about the relationship between magnetism and unconventional superconductivity. One explanation, motivated by the so-called bad-metal behaviour of these materials, proposes that magnetism and superconductivity develop out of quasi-localized magnetic moments which are generated by strong electron-electron correlations. Another suggests that these phenomena are the result of weakly interacting electron states that lie on nested Fermi surfaces. Here we address the issue by comparing the newly discovered alkaline iron selenide superconductors, which exhibit no Fermi-surface nesting, to their iron pnictide counterparts. We show that the strong-coupling approach leads to similar pairing amplitudes in these materials, despite their different Fermi surfaces. We also find that the pairing amplitudes are largest at the boundary between electronic localization and itinerancy, suggesting that new superconductors might be found in materials with similar characteristics.

cond-mat.supr-con↗

arXiv2013

Theory of asymmetric and negative differential magnon tunneling under temperature bias: Towards a spin Seebeck diode and transistor

We study the nonequilibrium transport for the asymmetric and negative differential magnon tunneling driven by temperature bias. We demonstrate that the many-body magnon interaction that makes the magnonic spectrum temperature-dependent is the crucial factor for the emergence of rectification and negative differential spin Seebeck effects in magnon tunneling junctions. When magnonic junctions have temperature-dependent density of states, reversing the temperature bias is able to give asymmetric spin currents and increasing temperature bias could give an anomalously decreasing magnonic spin current. We show that these properties are relevant for building spin Seebeck diodes and transistors, which could play important roles in controlling information and energy in magnonics and spin caloritronics.

cond-mat.mes-hall↗

arXiv2013

Disorder effects in multiorbital $s_{\pm}$-wave superconductors: Implications for Zn-doped BaFe$_2$As$_2$ compounds

Recent experiments on Zn-doped 122-type iron pnictides, Ba(Fe$_{1-x-y}$Co$_y$Zn$_x$)$_2$As$_2$, are challenging our understanding of electron doping the 122s and the interplay between doping and impurity scattering. To resolve this enigma, we investigate the disorder effects of nonmagnetic Zn impurities in the strong (unitary) scattering limit on various properties of the system in the $s_{\pm}$-wave superconducting pairing state. The lattice Bogoliubov-de Gennes equation (BdG) is solved self-consistently based on a minimal two-orbital model with an extended range of impurity concentrations. We find that Zn impurity is best modeled as a defect, where charge is mainly localized, but scattering is extended over a few lattice sites. With increasing Zn concentration the density of states shows a gradual filling of the gap, revealing the impurity-induced pair breaking effect. Moreover, both the disorder configuration-averaged superconducting order parameter and the superfluid density are dramatically suppressed towards the dirty limit, indicating the violation of the Anderson theorem for conventional s-wave superconductors and the breakdown of the Abrikosov-Gorkov theory for impurity-averaged Green's functions. Furthermore, we find that the superconducting phase is fully suppressed close to the critical impurity concentration of roughly $n_\text{imp}\approx 10%$, in agreement with recent experiments.

cond-mat.str-el↗

arXiv2013

Fermi surface topology and de Hass-van Alphen orbits in PuIn$_{\rm 3}$ and PuSn$_{\rm 3}$ compounds

Since the recent discovery of plutonium-based superconductors such as PuCoIn$_{\rm 5}$, systematic studies of the electronic properties for plutonium compounds are providing insight into the itinerancy-localization crossover of Pu 5$f$ electrons. We are particularly interested in understanding the Fermi surface properties of PuIn$_{3}$ compound, which serves as the building block for the PuCoIn$_{\rm 5}$ superconductor. Motivated by the first observation of quantum oscillation and renewed interest in the de-Hass van-Alphen (dHvA) measurements on PuIn$_{\rm 3}$, we study the Fermi surface (FS) topology and the band dispersion in both paramagnetic and antiferromagnetic state of PuIn$_{\rm 3}$, based on density functional theory with generalized gradient approximation. We compare the results with its isostructural paramagnetic compound PuSn$_{\rm 3}$. We show the detailed Fermi surfaces of compounds PuIn$_{\rm 3}$ and PuSn$_{\rm 3}$ and conclude that the FS topology of an antiferromagnetic PuIn$_3$ agrees better with dHvA measurements. In addition, we show the magnetization of the antiferromagnetic order can alter the angle dependence and values of the effective mass for the dHvA orbits. We point out the accurate determination of the magnetic order orientation with respect to the crystal orientation is crucial to advance the understanding of the electronic structures of the PuIn$_{\rm 3}$ compound.

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arXiv2013

Heat Diode Effect and Negative Differential Thermal Conductance across Nanoscale Metal-Dielectric Interfaces

Controlling heat flow by phononic nanodevices has received significant attention recently because of its fundamental and practical implications. Elementary phononic devices such as thermal rectifiers, transistors, and logic gates are essentially based on two intriguing properties: heat diode effect and negative differential thermal conductance. However, little is known about these heat transfer properties across metal-dielectric interfaces, especially at nanoscale. Here we analytically resolve the microscopic mechanism of the nonequilibrium nanoscale energy transfer across metal-dielectric interfaces, where the inelastic electron-phonon scattering directly assists the energy exchange. We demonstrate the emergence of heat diode effect and negative differential thermal conductance in nanoscale interfaces and explain why these novel thermal properties are usually absent in bulk metal-dielectric interfaces. These results will generate exciting prospects for the nanoscale interfacial energy transfer, which should have important implications in designing hybrid circuits for efficient thermal control and open up potential applications in thermal energy harvesting with low-dimensional nanodevices.

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arXiv2013

Quantum critical Kondo destruction in the Bose-Fermi Kondo model with a local transverse field

Recent studies of the global phase diagram of quantum-critical heavy-fermion metals prompt consideration of the interplay between the Kondo interactions and quantum fluctuations of the local moments alone. Toward this goal, we study a Bose-Fermi Kondo model (BFKM) with Ising anisotropy in the presence of a local transverse field that generates quantum fluctuations in the local-moment sector. We apply the numerical renormalization-group method to the case of a sub-Ohmic bosonic bath exponent and a constant conduction-electron density of states. Starting in the Kondo phase at zero transverse-field, there is a smooth crossover with increasing transverse field from a fully screened to a fully polarized impurity spin. By contrast, if the system starts in its localized phase, then increasing the transverse field causes a continuous, Kondo-destruction transition into the partially polarized Kondo phase. The critical exponents at this quantum phase transition exhibit hyperscaling and take essentially the same values as those of the BFKM in zero transverse field. The many-body spectrum at criticality varies continuously with the bare transverse field, indicating a line of critical points. We discuss implications of these results for the global phase diagram of the Kondo lattice model.

cond-mat.str-el↗

arXiv2013

Orbital-selective superconductivity, gap anisotropy and spin resonance excitations in a multiorbital t-J1-J2 model for iron pnictides

We study the orbital-dependent superconducting pairing in a five-orbital t-J1-J2 model for iron pnictides. Depending on the orbital selectivity of electron correlations and the orbital characters along the Fermi surface, the superconducting gap in an A_{1g} pairing state may exhibit anisotropy. This anisotropy varies with the degree of J1-J2 magnetic frustration. We have also calculated the dynamical spin susceptibility in the superconducting state. The frequency dependence of the susceptibility at the antiferromagnetic wavevector (π,0) shows a resonance, whose width is enhanced by the orbital dependence of the superconducting gap; when the latter is sufficiently strong, the resonance peak may be split into two. We discuss the implications of our results on the recent angle-resolved photoemission and neutron-scattering measurements in several superconducting iron pnictides.

cond-mat.supr-con↗

arXiv2013

Orbital-dependent effects of electron correlations in microscopic models for iron-based superconductors

The bad metal behavior in the normal state of the iron-based superconductors suggests an intimate connection between the superconductivity and a proximity to a Mott transition. At the same time, there is strong evidence for the multi-orbital nature of the electronic excitations. It is then important to understand the orbital-dependent effects of electron correlations. In this paper we review the recent theoretical progresses on the metal-to-insulator transition in multiorbital models for the iron-based superconductors. These include studies of models that contain at least the 3d xy and 3d xz/yz models, using a slave-spin technique. For commensurate filling corresponding to that of the parent iron pnictides and chalcogenideds, a Mott transition generally exists in all these models. Near the Mott transition, a strongly correlated metal exhibiting bad metal features and strong orbital selectivity is stabilized due to the interplay of Hund's coupling and orbital-degeneracy breaking. Particularly for the alkaline iron selenides, the ordered vacancies effectively reduce the kinetic energy, thereby pushing the system further into the Mott-insulating regime; in the metallic state, there exists an orbital-selective Mott phase in which the iron 3d xy orbital is Mott localized while the other 3d orbitals are still itinerant. An overall phase diagram for the alkaline iron selenides has been proposed, in which the orbital-selective Mott phase connects between the superconducting phase and the Mott-insulating parent state.

cond-mat.supr-con↗

arXiv2013

Anomalous Energy Transport across Topological Insulator Superconductor Junctions

We study the nonequilibrium energy transport across a topological insulator/superconductor junction, by deriving an interfacial heat current formula through scattering wave approach. Several anomalous thermal properties are uncovered, such as thermal energy's Klein tunneling, asymmetric Kapitza resistance and negative differential thermal resistance. We expect these findings could have potential applications for the energy control in various hybridized mesoscopic systems.

cond-mat.mes-hall↗

arXiv2013

Phase diagram of doped BaFe$_2$As$_2$ superconductor under broken $C_4$ symmetry

We develop a minimal multiorbital tight-binding model with realistic hopping parameters. The model breaks the symmetry of the tetragonal point group by lowering it from $C_4$ to $D_{2d}$, which accurately describes the Fermi surface evolution of the electron-doped BaFe$_{2-x}$Co$_x$As$_2$ and hole-doped Ba$_{1-y}$K$_y$Fe$_2$As$_2$ compounds. An investigation of the phase diagram with a mean-field $t$-$U$-$V$ Bogoliubov-de Gennes Hamiltonian results in agreement with the experimentally observed electron- and hole-doped phase diagram with only one set of $t$, $U$ and $V$ parameters. Additionally, the self-consistently calculated superconducting order parameter exhibits $s^\pm$-wave pairing symmetry with a small d-wave pairing admixture in the entire doping range, % The superconducting $s^\pm + d$-wave order parameter which is the subtle result of the weakly broken symmetry and competing interactions in the multiorbital mean-field Hamiltonian.

cond-mat.supr-con↗

arXiv2013

Singularity in self-energy and composite fermion excitations of interacting electrons

We propose that a composite fermion operator $f_{iσ}(2n_{i{\bar σ}}-1)$ could have coherent excitations, where $f_{iσ}$ is the fermion operator for interacting electrons and $n_{i{\bar σ}}$ is the number operator of the opposite spin. In the two-impurity Anderson model, it is found that the excitation of this composite fermion has a pseudogap in the Kondo regime, and has a finite spectral weight in the regime where the excitation of the regular fermion $f_{iσ}$ has a pseudogap. In the latter regime, the self-energy of $f_{iσ}$ is found to be singular near Fermi energy. We argue that this composite fermion could develop a Fermi surface with Fermi liquid behaviors but "hidden" from charge excitations in lattice generalizations. We further illustrate that this type of excitations is essential in addressing the pseudogap state and unconventional superconductivity.

cond-mat.str-el↗

arXiv2013

Terahertz conductivity of twisted bilayer graphene

Using terahertz time-domain spectroscopy, the real part of optical conductivity [$σ_{1}(ω)$] of twisted bilayer graphene was obtained at different temperatures (10 -- 300 K) in the frequency range 0.3 -- 3 THz. On top of a Drude-like response, we see a strong peak in $σ_{1} (ω)$ at $\sim$2.7 THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7 THz to an enhanced density of states at that energy, that is caused by the presence of a van Hove singularity arising from a commensurate twisting of the two graphene layers.

cond-mat.mes-hall↗

arXiv2013

The Electronic Correlation Strength of Pu

An electronic quantity, the correlation strength, is defined as a necessary step for understanding the properties and trends in strongly correlated electronic materials. As a test case, this is applied to the different phases of elemental Pu. Within the GW approximation we have surprisingly found a "universal" scaling relationship, where the f-electron bandwidth reduction due to correlation effects is shown to depend only on the local density approximation bandwidth and is otherwise independent of crystal structure and lattice constant.

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