arXiv subjects

Zhong-Yi Lu

Publications and source records attributed to Zhong-Yi Lu.

At least 199 records · Page 11Linked to original sources

arXiv2013

Random site dilution properties of frustrated magnets on a hierarchical lattice

We present a method to analyze magnetic properties of frustrated Ising spin models on specific hierarchical lattices with random dilution. Disorder is induced by dilution and geometrical frustration rather than randomness in the internal couplings of the original Hamiltonian. The two-dimensional model presented here possesses a macroscopic entropy at zero temperature in the large size limit, very close to the Pauling estimate for spin-ice on pyrochlore lattice, and a crossover towards a paramagnetic phase. The disorder due to dilution is taken into account by considering a replicated version of the recursion equations between partition functions at different lattice sizes. An analysis at first order in replica number allows for a systematic reorganization of the disorder configurations, leading to a recurrence scheme. This method is numerically implemented to evaluate the thermodynamical quantities such as specific heat and susceptibility in an external field.

cond-mat.dis-nn↗

arXiv2013

Prediction of phonon-mediated high temperature superconductivity in stoichiometric Li$_2$B$_3$C

The discovery of superconductivity in Magnesium Diborate (MgB$_2$) has stimulated great interest in the search of new superconductors with similar lattice structures. Unlike cuprate or iron-based superconductors, MgB$_2$ is indisputably a phonon-mediated high temperature superconductor. The emergence of high temperature superconductivity in this material results from the strong coupling between the boron $σ$-bonding electrons around the Fermi level and the bond-stretching optical phonon modes. Here we show, based on the first-principles calculations, that Li$_2$B$_3$C is such a good candidate of superconductor whose superconducting transition temperature (T$_c$) might be even higher than MgB$_2$. Li$_2$B$_3$C consists of alternating graphene-like boron-carbon layers and boron-boron layers with intercalated lithium atoms between them. Similar to MgB$_2$, Li$_2$B$_3$C is inherently metallic and possesses two $σ$- and two $π$-electron bands around the Fermi energy. The superconducting pairs are glued predominately by the strong interaction between boron $σ$-bonding electrons and various optical phonon modes.

cond-mat.supr-con↗

arXiv2012

Layered pnictide-oxide Na$_2$Ti$_2$Pn$_2$O (Pn=As, Sb): a paradigm for spin density waves

From the first-principles calculations, we have studied the electronic and magnetic structures of compound Na$_2$Ti$_2$Pn$_2$O (Pn = As or Sb). We find that in the ground state Na$_2$Ti$_2$As$_2$O is a blocked checkerboard antiferromagnetic semiconductor with a small band gap of about 0.15 eV, in contrast, Na$_2$Ti$_2$Sb$_2$O is a bi-collinear antiferromagnetic semimetal, both with a small moment of about 0.5$μ_B$ around each Ti atom. We show that there is a strong Fermi surface nesting in Na$_2$Ti$_2$Pn$_2$O. And we verify that the blocked checkerboard and bi-collinear antiferromagnetic states both are the spin density waves induced by the Fermi surface nesting. A tetramer structural distortion is found in company with the formation of a blocked checkerboard antiferromagnetic order, in good agreement with the experimentally observed commensurate structural distortion but with space group symmetry retained after the anomaly happening. Further analysis and discussion in connection with experimental observations are given as well.

cond-mat.supr-con↗

arXiv2011

Cluster dynamical mean field theory of quantum phases on a honeycomb lattice

We have studied the ground state of the half-filled Hubbard model on a honeycomb lattice by performing the cluster dynamical mean field theory calculations with exact diagonalization on the cluster-impurity solver. Through using elaborate numerical analytic continuation, we identify the existence of a `spin liquid' from the on-site interaction U=0 to $U_c$ (between $4.6t$ and $4.85t$) with a smooth crossover correspondingly from the charge fluctuation dominating phase into the charge correlation dominating phase. The semi-metallic state exits only at U=0. We further find that the magnetic phase transition at $U_c$ from the `spin liquid' to the Néel antiferromagnetic Mott insulating phase is a first-order quantum phase transition. We also show that the charge fluctuation plays a substantial role on keeping the `spin liquid' phase against the emergence of a magnetic order.

cond-mat.str-el↗

arXiv2011

Exact results for intrinsic electronic transport in graphene

We present exact results for the electronic transport properties of graphene sheets connected to two metallic electrodes. Our results, obtained by transfer-matrix methods, are valid for all sheet widths and lengths. In the limit of large width-to-length ratio relevant to recent experiments, we find a Dirac-point conductivity of $2e^2/\sqrt{3}h$ and a sub-Poissonian Fano factor of $2 - 3\sqrt{3}/π\simeq 0.346$ for armchair graphene; for the zigzag geometry these are respectively 0 and 1. Our results reflect essential effects from both the topology of graphene and the electronic structure of the leads, giving a complete microscopic understanding of the unique intrinsic transport in graphene.

cond-mat.mes-hall↗

arXiv2011

First-principles study of pressure-induced magnetic phase transitions in ternary iron selenide K$_{0.8}$Fe$_{1.6}$Se$_2$

We have studied the pressure effect on electronic structures and magnetic orders of ternary iron selenide K$_{0.8}$Fe$_{1.6}$Se$_2$ by the first-principles electronic structure calculations. At low pressure, the compound is in the blocked checkerboard antiferromagnetic (AFM) semiconducting phase, as observed by the neutron scatting measurements. Applying pressure induces two phase transitions, first from the blocked checkerboard AFM semiconducting phase to a collinear AFM metallic phase around 12 GPa, and then to a non-magnetic metallic phase around 25 GPa, respectively. Our results help to clarify the recent experimental measurements under pressure.

cond-mat.supr-con↗

arXiv2010

The layered compound CaClFeP is an Arsenic-free high $T_c$ iron-pnictide

We first analyze why the iron pnictides with high $T_c$ superconductivity so far are As-based, by the Hund's rule correlation picture, then examine the P-based and Sb-based cases, respectively. Consequently, we propose that CaClFeP with ZrCuSiAs-type structure is an As-free high $T_c$ iron-pnictide. The subsequent density functional theory calculations show that the ground state of CaClFeP is of a collinearly antiferromagnetic order on Fe moments with structural distortion, resulting from the interplay between the strong nearest and next-nearest neighbor antiferromagnetic superexchange interactions bridged by P atoms, similar as the As-based pnictides. The other P-based pnictides are either nonmagnetic or magnetic but with weak exchange interactions. The Sb-based pnictides unlikely show high $T_c$ superconductivity because of the existence of robust ferromagnetic order.

cond-mat.mtrl-sci↗

arXiv2010

Pnicogen-bridged antiferromagnetic superexchange interactions in iron pnictides

The first-principles electronic structure calculations made substantial contribution to study of high $T_c$ iron-pnictide superconductors. By the calculations, LaFeAsO was first predicted to be an antiferromagnetic semimetal, and the novel bi-collinear antiferromagnetic order was predicted for $α$-FeTe. Moreover, based on the calculations the underlying mechanism was proposed to be Arsenic-bridged antiferromagnetic superexchange interaction between the next-nearest neighbor Fe moments. In this article, this physical picture is further presented and discussed in association with the elaborate first-principles calculations on LaFePO. The further discussion of origin of the magnetism in iron-pnictides and in connection with superconductivity is presented as well.

cond-mat.mtrl-sci↗

arXiv2010

Pressure- and temperature-induced structural phase transitions of CaFe$_2$As$_2$ and BaFe$_2$As$_2$ studied in the Hund's rule correlation picture

With the proposed Hund's rule correlation picture, i.e. the fluctuating Fe local moments with the As-bridged antiferromagnetic superexchange interactions, the exceptional collapsed tetragonal phase and related phase transitions observed in CaFe$_2$As$_2$ are well understood. With the same framework, a pressure-temperature phase diagram is predicted for BaFe$_2$As$_2$ as well, in which a paramagnetic tetragonal and a collinear antiferromagnetic orthorhombic structures to nonmagnetic tetragonal structure transitions take place around 4-8 GPa and 10-15 GPa respectively, and a nonmagnetic tetragonal to a nonmagnetic collapsed tetragonal structure transition takes place over 26 GPa. Our study helps better understand the complex correlation among crystal structure, magnetism, and electronic structure in pnictides, a precondition to understand the superconductivity in pnictides.

cond-mat.mtrl-sci↗

arXiv2010

Electronic structures of ternary iron arsenides AFe$_2$As$_2$ (A=Ba, Ca, or Sr)

We have studied the electronic and magnetic structures of the ternary iron arsenides AFe$_2$As$_2$ (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions bridged by As $4p$ orbitals. The correction from the spin-orbit interaction to the band structure is small. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based on the calculations, we propose that the low energy dynamics of these materials is described effectively by a $t-J_H-J_1-J_2$-type model.

cond-mat.mtrl-sci↗

arXiv2010

Electronic structure of Fe1.04(Te0.66Se0.34)

We report the electronic structure of the iron-chalcogenide superconductor, Fe1.04(Te0.66Se0.34), obtained with high resolution angle-resolved photoemission spectroscopy and density functional calculations. In photoemission measurements, various photon energies and polarizations are exploited to study the Fermi surface topology and symmetry properties of the bands. The measured band structure and their symmetry characters qualitatively agree with our density function theory calculations of Fe(Te0.66Se0.34), although the band structure is renormalized by about a factor of three. We find that the electronic structures of this iron-chalcogenides and the iron-pnictides have many aspects in common, however, significant differences exist near the Gamma-point. For Fe1.04(Te0.66Se0.34), there are clearly separated three bands with distinct even or odd symmetry that cross the Fermi energy (EF) near the zone center, which contribute to three hole-like Fermi surfaces. Especially, both experiments and calculations show a hole-like elliptical Fermi surface at the zone center. Moreover, no sign of spin density wave was observed in the electronic structure and susceptibility measurements of this compound.

cond-mat.str-el↗

arXiv2009

$π$-junction to probe antiphase s-wave pairing in iron pnictide superconductors

Josephson junctions between a $FeAs$-based superconductor with antiphase s-wave pairing and a conventional s-wave superconductor are studied. The translational invariance in a planar junction between a single crystal pnictide and an aluminum metal greatly enhances the relative weight of electron pockets in the pnictide to the critical current. In a wide doping region of the pnictide, a planar and a point contact junctions have opposite phases, which can be used to design a tri-junction ring with $π$ phase to probe the antiphase pairing.

cond-mat.supr-con↗

arXiv2009

Atomic and electronic structures of ternary iron arsenides $A$Fe$_2$As$_2$(001) surfaces ($A$=Ba, Sr, or Ca)

By the first-principles electronic structure calculations, we find that energetically the most favorable cleaved $A$Fe$_2$As$_2$(001) surface ($A$=Ba, Sr, or Ca) is $A$-terminated with a $(\sqrt{2}\times \sqrt{2})R45^{\circ}$ or $(1\times 2)$ order. The $(1\times 2)$ ordered structure yields a $(1\times 2)$ dimerized STM image, in agreement with the experimental observation. The $A$ atoms are found to diffuse on the surface with a small energy barrier so that the cleaving process may destroy the $A$ atoms ordering. At the very low temperatures this may result in an As-terminated surface with the $A$ atoms in randomly assembling. The As-terminated BaFe$_2$As$_2$ surface in orthorhombic phase is $(\sqrt{2}\times\sqrt{2})R45^{\circ}$ buckled, giving rise to a switchable $(\sqrt{2}\times \sqrt{2})R45^{\circ}$ STM pattern upon varying the applied bias. No any reconstruction is found for the other As-terminated surfaces. There are surface states crossing or nearby the Fermi energy in the As-terminated and $(1\times 2)$ $A$-terminated surfaces. A unified physical picture is thus established to help understand the cleaved $A$Fe$_2$As$_2$(001) surfaces.

cond-mat.mtrl-sci↗

arXiv2009

Universal linear-temperature dependence of static magnetic susceptibility in iron-pnictides

A universal linear-temperature dependence of the uniform magnetic susceptibility has been observed in the nonmagnetic normal state of iron-pnictides. This non-Pauli and non-Curie-Weiss-like paramagnetic behavior cannot be understood within a pure itinerant picture. We argue that it results from the existence of a wide antiferromagnetic fluctuation window in which the local spin-density-wave correlations exist but the global directional order has not been established yet.

cond-mat.str-el↗

arXiv2008

Antiferromagnetic superexchange interactions in LaOFeAs

From first-principles calculations, we have studied the electronic and magnetic structures of the ground state of LaOFeAs. The Fe spins are found to be collinear antiferromagnetic ordered, resulting from the interplay between the strong nearest and next-nearest neighbor superexchange antiferromagnetic interactions. The structure transition observed by neutron scattering is shown to be magnetically driven. Our study suggests that the antiferromagnetic fluctuation plays an important role in the Fe-based superconductors. This sheds light on the understanding of the pairing mechanism in these materials.

cond-mat.mtrl-sci↗

arXiv2008

Iron-based layered superconductor LaO$_{1-x}$F$_x$FeAs: an antiferromagnetic semimetal

We have studied the newly found superconductor compound LaO$_{1-x}$F$_x$FeAs through the first-principles density functional theory calculations. We find that the parent compound LaOFeAs is a quasi-2-dimensional antiferromgnetic semimetal with most carriers being electrons and with a magnetic moment of $2.3μ_B$ located around each Fe atom on the Fe-Fe square lattice. Furthermore this is a commensurate antiferromagnetic spin density wave due to the Fermi surface nesting, which is robust against the F-doping. The observed superconduction happens on the Fe-Fe antiferromagnetic layer, suggesting a new superconductivity mechanism, mediated by the spin fluctuations. An abrupt change on the Hall measurement is further predicted for the parent compound LaOFeAs.

cond-mat.mtrl-sci↗

arXiv2004

Spin-dependent resonant tunneling through quantum-well states in magnetic metallic thin films

Quantum-well (QW) states in {\it nonmagnetic} metal layers contained in magnetic multilayers are known to be important in spin-dependent transport, but the role of QW states in {\it magnetic} layers remains elusive. Here we identify the conditions and mechanisms for resonant tunneling through QW states in magnetic layers and determine candidate structures. We report first-principles calculations of spin-dependent transport in epitaxial Fe/MgO/FeO/Fe/Cr and Co/MgO/Fe/Cr tunnel junctions. We demonstrate the formation of sharp QW states in the Fe layer and show discrete conductance jumps as the QW states enter the transport window with increasing bias. At resonance, the current increases by one to two orders of magnitude. The tunneling magnetoresistance ratio is several times larger than in simple spin tunnel junctions and is positive (negative) for majority- (minority-) spin resonances, with a large asymmetry between positive and negative biases. The results can serve as the basis for novel spintronic devices.

cond-mat.mes-hall↗

arXiv1995

Atomic and electronic structure of ideal and reconstructed $α$-Sn (111) surface

We have carried out an ab-initio study of $α$-Sn (111), with the aim of predicting and understanding its structure, reconstructions, and electronic states. We consider a variety of structural possibilities, and optimize them by moving atoms according to Hellmann-Feynman forces. Our results indicate that the unreconstructed surface is highly unstable, while a variety of reconstructions compete for the true ground state. Extrapolated trends from diamond to Si to Ge are well borne out, with a $2\times 1 π$-bonded chain reconstruction prevailing in the absence of adatoms, and a $c(4\times 2)$ or $(2\times 2)$ basic adatom-restatom unit reconstruction otherwise. Accompanying surface bucklings are in both cases larger than in Si and Ge, with consequently large ionic charge transfers predicted. Search for a $β$-Sn-like metallic state of the surface turned out to be inconclusive.

cond-mat↗