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Jianhui Dai

Publications and source records attributed to Jianhui Dai.

At least 73 records · Page 4Linked to original sources
arXiv2011

Block Spin Ground State and 3-Dimensionality of (K,Tl)Fe$_{1.6}$Se$_2$

The magnetic properties and electronic structure of (K,Tl)y Fe1.6 Se2 is studied using first-principles calculations. The ground state is checkerboard antiferromagnetically coupled blocks of the minimal Fe4 squares, with a large block spin moment ~11.2μB . The magnetic interactions could be modelled with a simple spin model involving both the inter- and intra-block, as well as the n.n. and n.n.n. couplings. The calculations also suggest a metallic ground state except for y = 0.8 where a band gap ~400 - 550 meV opens, showing an antiferromagnetic insulator ground state for (K,Tl)0.8 Fe1.6 Se2 . The electronic structure of the metallic (K,Tl)y Fe1.6 Se2 is highly 3-dimensional with unique Fermi surface structure and topology. These features indicate that the Fe-vacancy ordering is crucial to the physical properties of (K,Tl)y Fe2-x Se2 .

cond-mat.supr-con↗
arXiv2011

Electronic Structure of KFe$_2$Se$_2$ from First Principles Calculations

Electronic structure and magnetic properties for iron-selenide KFe$_2$Se$_2$ are studied by first-principles calculations. The ground state is stripe-like antiferromagnetic with calculated 2.26 $μ_B$ magnetic moment on Fe atoms; and the $J_1$, $J_2$ coupling strengths are calculated to be 0.038 eV and 0.029 eV. The states around $E_F$ are dominated by the Fe-3d orbitals which hybridize noticeably to the Se-4p orbitals. While the band structure of KFe$_2$Se$_2$ is similar to a heavily electron-doped BaFe$_2$As$_2$ or FeSe system, the Fermi surface of KFe$_2$Se$_2$ is much closer to \fs11 system since the electron sheets around $M$ is symmetric with respect to $x$-$y$ exchange. These features, as well as the absence of Fermi surface nesting, suggest that the parental KFe$_2$Se$_2$ could be regarded as an electron over-doped 11 system with possible local moment magnetism.

cond-mat.supr-con↗
arXiv2011

Topological insulators with perfect vacancy superstructure and possible implications for iron chalcogenide superconductors

Motivated by the newly-discovered intercalated iron chalcogenide superconductors, we construct a single orbital tight-binding model for topological insulators on the square lattice with a perfect vacancy superstructure. We find that such lattice structure naturally accommodates a non-vanishing geometry phase associated with the next-nearest-neighbor spin-orbit interaction. By calculating the bulk band structures and the finite stripe edge states, we show that the topological insulator phases can be tuned at certain electron fillings in a wide range of the model parameters. The possible implications of these results for the iron deficient compounds $(A,Tl)_{y}Fe_{2-x}Se_2$ have been discussed.

cond-mat.str-el↗
arXiv2011

CeNiAsO: an antiferromagnetic dense Kondo lattice

A cerium containing pnictide, CeNiAsO, crystallized in the ZrCuSiAs type structure, has been investigated by measuring transport and magnetic properties, as well as specific heat. We found that CeNiAsO is an antiferromagnetic dense Kondo lattice metallic compound with Kondo scale $T_K \sim$ 15 K and shows an enhanced Sommerfeld coefficient of $γ_0 \sim$ 203 mJ/mol$\cdot$K$^{2}$. While no superconductivity can been observed down to 30 mK, Ce ions exhibit two successive antiferromagnetic (AFM) transitions. We propose that the magnetic moment of Ce ion could align in the G type AFM order below the first transition at $T_{N1}$=9.3 K, and it might be modified into the C type AFM order below a lower transition at $T_{N2}$=7.3 K. Our results indicate that the 3$d-4f$ interlayer Kondo interactions play an important role in Ni-based Ce-containing pnictide.

cond-mat.str-el↗
arXiv2011

Electronic Structure and Mott Localization in Iron Deficient TlFe$_{1.5}$Se$_2$ with Superstructures

Electronic structure and magnetic properties for iron deficient TlFe$_{2-x}$Se$_2$ compounds are studied by first-principles calculations. We find that for the case of $x=0.5$ with a Fe-vacancy ordered orthorhombic superstructure, the ground state exhibits a stripe-like antiferromagnetic ordering and opens a sizable band gap if the short-ranged Coulomb interaction of Fe-3d electrons is moderately strong, manifesting a possible Mott insulating state. While increasing Fe-vacancies from the $x=0$ side, where the band structure is similar to that of a heavily electron-doped FeSe system, the Mott localization can be driven by kinetic energy reduction as evidenced by the band narrowing effect. Implications of this scenario in the recent experiments on TlFe$_{2-x}$Se$_2$ are discussed.

cond-mat.supr-con↗
arXiv2010

Band Narrowing and Mott Localization in Iron Oxychalcogenides La2O2Fe2O(Se,S)2

Bad metal properties have motivated a description of the parent iron pnictides as correlated metals on the verge of Mott localization. What has been unclear is whether interactions can push these and related compounds to the Mott insulating side of the phase diagram. Here we consider the iron oxychalcogenides La2O2Fe2O(Se,S)2, which contain an Fe square lattice with an expanded unit cell. We show theoretically that they contain enhanced correlation effects through band narrowing compared to LaOFeAs, and we provide experimental evidence that they are Mott insulators with moderate charge gaps. We also discuss the magnetic properties in terms of a Heisenberg model with frustrating J1-J2-J2' exchange interactions on a "doubled" checkerboard lattice.

cond-mat.str-el↗
arXiv2010

Phase diagram of CeFeAs$_{1-x}$P$_{x}$O obtained from electric resistivity, magnetization, and specific heat measurements

We performed a systematic study on the properties of CeFeAs$_{1-x}$P$_{x}$O ($0\leq x\leq 1$) by electrical resistivity, magnetization and specific heat measurements. The c-axis lattice constant decreases significantly with increasing P content, suggesting a remarkable chemical pressure. The Fe-3d electrons show the enhanced metallic behavior upon P-doping and undergo a magnetic quantum phase transition around $x \approx 0.4$. Meanwhile, the Ce-4f electrons develop a ferromagnetic order near the same doping level. The ferromagnetic order is vanishingly small around $x=0.9$. The data suggest a heavy-fermion-like behavior as $x\geq 0.95$. No superconductivity is observed down to 2 K. Our results show the ferromagnetic ordered state as an intermediate phase intruding between the antiferromagnetic bad metal and the nonmagnetic heavy fermion metal and support the cerium-containing iron pnictides as a unique layered Kondo lattice system.

cond-mat.str-el↗
arXiv2009

Superconductivity up to 30 K in the vicinity of quantum critical point in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$

We report bulk superconductivity induced by an isovalent doping of phosphorus in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. The P-for-As substitution results in shrinkage of lattice, especially for the FeAs block layers. The resistivity anomaly associated with the spin-density-wave (SDW) transition in the undoped compound is gradually suppressed by the P doping. Superconductivity with the maximum $T_c$ of 30 K emerges at $x$=0.32, coinciding with a magnetic quantum critical point (QCP) which is evidenced by the disappearance of SDW order and the linear temperature-dependent resistivity in the normal state. The $T_c$ values were found to decrease with further P doping, and no superconductivity was observed down to 2 K for $x\geq$ 0.77. The appearance of superconductivity in the vicinity of QCP hints to the superconductivity mechanism in iron-based arsenides.

cond-mat.supr-con↗
arXiv2009

Superconductivity in LaFeAs$_{1-x}$P$_{x}$O: effect of chemical pressures and bond covalency

We report the realization of superconductivity by an isovalent doping with phosphorus in LaFeAsO. X-ray diffraction shows that, with the partial substitution of P for As, the Fe$_2$As$_2$ layers are squeezed while the La$_2$O$_2$ layers are stretched along the c-axis. Electrical resistance and magnetization measurements show emergence of bulk superconductivity at $\sim$10 K for the optimally-doped LaFeAs$_{1-x}$P$_{x}$O ($x=0.25\sim0.3$). The upper critical fields at zero temperature is estimated to be 27 T, much higher than that of the LaFePO superconductor. The occurrence of superconductivity is discussed in terms of chemical pressures and bond covalency.

cond-mat.supr-con↗
arXiv2009

The f-spin physics of rare-earth iron pnictides: influence of d-electron antiferromagnetic order on the heavy fermion phase diagram

Some of the high Tc iron pnictides contain magnetic rare-earth elements, raising the question of how the existence and tunability of a d-electron antiferromagnetic order influences the heavy fermion behavior of the f-moments. With CeOFeP and CeOFeAs in mind as prototypes, we derive an extended Anderson lattice model appropriate for these quaternary systems. We show that the Kondo screening of the f-moments are efficiently suppressed by the d-electron ordering. We also argue that, inside the d-electron ordered state (as in CeOFeAs), the f-moments provide a rare realization of a quantum frustrated magnet with competing J1-J2-J3 interactions in an effective square lattice. Implications for the heavy fermion physics in broader contexts are also discussed.

cond-mat.str-el↗
arXiv2009

Exact solutions of a class of S=1 quantum Ising spin models

We propose a hole decomposition scheme to exactly solve a class of spin-1 quantum Ising models with transverse or longitudinal single-ion anisotropy. In this scheme, the spin-1 model is mapped onto a family of the $S=1/2$ transverse Ising models, characterized by the total number of holes. A recursion formula is derived for the partition function based on the reduced $S=1/2$ Ising model. This simplifies greatly the summation over all the hole configurations. It allows the thermodynamic quantities to be rigorously determined in the thermodynamic limit. The ground state phase diagram is determined for both the uniform and dimerized spin chains. The corresponding thermodynamic properties are calculated and discussed.

cond-mat.str-el↗
arXiv2009

Correlation effects in the iron pnictides

One of the central questions about the iron pnictides concerns the extent to which their electrons are strongly correlated. Here we address this issue through the phenomenology of the charge transport and dynamics, single-electron excitation spectrum, and magnetic ordering and dynamics. We outline the evidence that the parent compounds, while metallic, have electron interactions that are sufficiently strong to produce incipient Mott physics. In other words, in terms of the strength of electron correlations compared to the kinetic energy, the iron pnictides are closer to intermediately-coupled systems lying at the boundary between itinerancy and localization, such as V2O3 or Se-doped NiS2, rather than to simple antiferromagnetic metals like Cr. This level of electronic correlations produces a new small parameter for controlled theoretical analyses, namely the fraction of the single-electron spectral weight that lies in the coherent part of the excitation spectrum. Using this expansion parameter, we construct the effective low-energy Hamiltonian and discuss its implications for the magnetic order and magnetic quantum criticality. Finally, this approach sharpens the notion of magnetic frustration for such a metallic system, and brings about a multiband matrix t-J1-J2 model for the carrier-doped iron pnictides.

cond-mat.supr-con↗
arXiv2009

Narrow superconducting window in Ni-doped LaFeAsO

We have studied Ni-substitution effect in LaFe$_{1-x}$Ni$_{x}$AsO ($0\leq x \leq0.1$) by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. The nickel doping drastically suppresses the resistivity anomaly associated with spin-density-wave ordering in the parent compound. Superconductivity emerges in a narrow region of $0.03\leq x \leq0.06$ with the maximum $T_c$ of 6.5 K at $x$=0.04, where enhanced magnetic susceptibility shows up. The upper critical field at zero temperature is estimated to exceed the Pauli paramagnetic limit. The much lowered $T_c$ in comparison with LaFeAsO$_{1-x}$F$_{x}$ system is discussed.

cond-mat.supr-con↗
arXiv2009

Iron pnictides as a new setting for quantum criticality

Two major themes in the physics of condensed matter are quantum critical phenomena and unconventional superconductivity. These usually occur in the context of competing interactions in systems of strongly-correlated electrons. All this interesting physics comes together in the behavior of the recently discovered iron pnictide compounds that have generated enormous interest because of their moderately high-temperature superconductivity. The ubiquity of antiferromagnetic ordering in their phase diagrams naturally raises the question of the relevance of magnetic quantum criticality, but the answer remains uncertain both theoretically and experimentally. Here we show that the undoped iron pnictides feature a novel type of magnetic quantum critical point, which results from a competition between electronic localization and itinerancy. Our theory provides a mechanism to understand the experimentally-observed variation of the ordered moment among the undoped iron pnictides. We suggest P substitution for As in the undoped iron pnictides as a means to access this new example of magnetic quantum criticality in an unmasked fashion. Our findings point to the iron pnictides as a much-needed new setting for quantum criticality, one that offers a new set of control parameters.

cond-mat.supr-con↗
arXiv2009

Possible Kondo effect in the iron arsenides

The normal state of the iron arsenides shows the poor metallic behavior mixed with strong magnetic fluctuations. In particular, some FeAs-1111 and FeAs-122 compounds show the linear-T dependence of susceptibility above the spin-density wave (SDW) transition and the logarithmic upturn of resistivity at low temperatures. We suggest that this is due to the spin-flip scattering between the charge carriers and the local moments in the undoped FeAs layer where Kondo effect coexists with the SDW. This scenario is also accounted for the change of the magnetoresistance from positive to negative in the Sr3Sc2O5Fe2As2 compound.

cond-mat.str-el↗
arXiv2009

Superconductivity induced by phosphorus doping and its coexistence with ferromagnetism in EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$

We have studied EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$ by the measurements of x-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility, magnetoresistance and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chemical pressure to the system. It is found that EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$ undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of Eu$^{2+}$ moments at 20 K. This finding is the first observation of superconductivity stabilized by internal chemical pressure, and supplies a rare example showing coexistence of superconductivity and ferromagnetism in the ferro-arsenide family.

cond-mat.supr-con↗
arXiv2008

Rigorous Solution of the Spin-1 Quantum Ising Model with Single-ion Anisotropy

We solve the spin-1 quantum Ising model with single-ion anisotropy by mapping it onto a series of segmented spin-1/2 transverse Ising chains, separated by the $S^z =0$ states called holes. A recursion formula is derived for the partition function to simplify the summation of hole configurations. This allows the thermodynamic quantities of this model to be rigorously determined in the thermodynamic limit. The low temperature behavior is governed by the interplay between the hole excitations and the fermionic excitations within each spin-1/2 Ising segment. The quantum critical fluctuations around the Ising critical point of the transverse Ising model are strongly suppressed by the hole excitations.

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