arXiv subjects

Jiangping Hu

Publications and source records attributed to Jiangping Hu.

At least 235 records · Page 13Linked to original sources

arXiv2012

Effect of Li-deficiency impurities on the electron-overdoped LiFeAs superconductor

We use transport, inelastic neutron scattering, and angle resolved photoemission experiments to demonstrate that the stoichiometric LiFeAs is an intrinsically electron-overdoped superconductor similar to those of the electron-overdoped NaFe1-xTxAs and BaFe2-xTxAs2 (T = Co,Ni). Furthermore, we show that although transport properties of the stoichiometric superconducting LiFeAs and Li-deficient nonsuperconducting Li1-xFeAs are different, their electronic and magnetic properties are rather similar. Therefore, the nonsuperconducting Li1-xFeAs is also in the electron overdoped regime, where small Li deficiencies near the FeAs octahedra can dramatically suppress superconductivity through the impurity scattering effect.

cond-mat.supr-con↗

arXiv2012

Oriented gap opening in the magnetically ordered state of Iron-pnicitides: an impact of intrinsic unit cell doubling on the $Fe$ square lattice by $As$ atoms

We show that the complicated band reconstruction near Fermi surfaces in the magnetically ordered state of iron-pnictides observed by angle-resolved photoemission spectroscopies (ARPES) can be understood in a meanfield level if the \emph{intrinsic unit cell doubling} due to As atoms is properly considered as shown in the recently constructed S$_{4}$ microscopic effective model. The (0,$π$) or ($π$,0) col-linear antiferromagnetic (C-AFM) order does not open gaps between two points at Fermi surfaces linked by the ordered wave vector but forces a band reconstruction involving four points in unfolded Brillouin zone (BZ) and gives rise to small pockets or hot spots. The S$_4$ symmetry naturally chooses a staggered orbital order over a ferro-orbital order to coexist with the C-AFM order. These results strongly suggest that the kinematics based on the S$_{4}$ symmetry captures the essential low energy physics of iron-based superconductors.

cond-mat.supr-con↗

arXiv2012

$S_4$ Symmetric Microscopic Model for Iron-Based Superconductors

Although iron-based superconductors are multi-orbital systems with complicated band structures, we demonstrate that the low energy physics which is responsible for high-$T_c$ superconductivity is essentially governed by one effective Hamiltonianwith two almost decoupled orbitals near half filling. This underlining electronic structure is protected by the $S_4$ symmetry. With repulsive or strong next nearest neighbor antiferromagnetic exchange interactions, each single-orbital effective Hamiltonian results in a robust $A_{1g}$ s-wave pairing which can be exactly mapped to the d-wave pairing observed in cuprates. The classification of the superconducting(SC) states according to the $S_4$ symmetry leads to a natural prediction of the existence of two different phases named A and B phases. In the B phase, the superconducting order has an overall sign change along c-axis between the top and bottom As(Se) planes in a single Fe-(As)Se trilayer structure, which is an analogy of the sign change under the $90^\circ$ degree rotation in the d-wave SC state of cuprates. Our derivation provides a unified understanding of iron-pnictides and iron-chalcogenides, and suggests that cuprates and iron-based superconductors share identical high-$T_c$ superconducting mechanism.

cond-mat.supr-con↗

arXiv2012

Controlling phase separation of a two-component Bose-Einstein condensate by confinement

We point out that the widely accepted condition g11g22<g122 for phase separation of a two-component Bose-Einstein condensate is insufficient if kinetic energy is taken into account, which competes against the intercomponent interaction and favors phase mixing. Here g11, g22, and g12 are the intra- and intercomponent interaction strengths, respectively. Taking a d-dimensional infinitely deep square well potential of width L as an example, a simple scaling analysis shows that if d=1 (d=3), phase separation will be suppressed as L\rightarrow0 (L\rightarrow\infty) whether the condition g11g22<g122 is satisfied or not. In the intermediate case of d=2, the width L is irrelevant but again phase separation can be partially or even completely suppressed even if g11g22<g122. Moreover, the miscibility-immiscibility transition is turned from a first-order one into a second-order one by the kinetic energy. All these results carry over to d-dimensional harmonic potentials, where the harmonic oscillator length ξho plays the role of L. Our finding provides a scenario of controlling the miscibility-immiscibility transition of a two-component condensate by changing the confinement, instead of the conventional approach of changing the values of the g's.

cond-mat.quant-gas↗

arXiv2012

Iron-Based Superconductors: $S_4$ Symmetry Superconductors

I elaborate the construction of the recent two-orbital model for iron-based superconductors\cite{hu-hao} and discuss the properties of superconducting(SC) phases based on the $S_4$ symmetry. I demonstrate further that the underlining electronic structure is an almost decoupled two-orbital model obeying the $S_4$ symmetry. I discuss the classification of the SC states according to the $S_4$ symmetry, which leads to a natural prediction that there are two different phases even in the proposed $A_{1g}$ s-wave SC state of iron-based superconductors and the sign change in the SC state should be hidden between the upper and lower As(Se) layer in a single Fe-(As)Se layer, an analogy of the sign change under the $90^\circ$ degree rotation in the d-wave SC state of cuprates. The $S_4$ symmetry makes iron-based superconductors a unique class of superconductors with promising device applications.

cond-mat.supr-con↗

arXiv2012

One-loop renormalization group study of boson-fermion mixtures

A weakly interacting boson-fermion mixture model was investigated using Wisonian renormalization group analysis. This model includes one boson-boson interaction term and one boson-fermion interaction term. The scaling dimensions of the two interaction coupling constants were calculated as 2-D at tree level and the Gell-Mann-Low equations were derived at one-loop level. We find that in the Gell-Mann-Low equations the contributions from the fermion loops go to zero as the length scale approaches infinity. After ignoring the fermion loop contributions two fixed points were found in 3 dimensional case. One is the Gaussian fixed point and the other one is Wilson-Fisher fixed point. We find that the boson-fermion interaction decouples at the Wilson-Fisher fixed point. We also observe that under RG transformation the boson-fermion interaction coupling constant runs to negative infinity with a small negative initial value, which indicates a boson-fermion pairing instability. Furthermore, the possibility of emergent supersymmetry in this model was discussed.

cond-mat.quant-gas↗

arXiv2012

Magnetic and Orbital Orders Coupled to Negative Thermal Expansion in Mott Insulators, Ca2Ru1-xMxO4 (M = Mn and Fe)

Ca2RuO4 is a structurally-driven Mott insulator with a metal-insulator transition at TMI = 357K, followed by a well-separated antiferromagnetic order at TN = 110 K. Slightly substituting Ru with a 3d transition metal ion M effectively shifts TMI by weakening the orthorhombic distortion and induces either metamagnetism or magnetization reversal below TN. Moreover, M doping for Ru produces negative thermal expansion in Ca2Ru1-xMxO4 (M = Cr, Mn, Fe or Cu); the lattice volume expands on cooling with a total volume expansion ratio, ΔV/V, reaching as high as 1%. The onset of the negative thermal expansion closely tracks TMI and TN, sharply contrasting classic negative thermal expansion that shows no relevance to electronic properties. In addition, the observed negative thermal expansion occurs near room temperature and extends over a wide temperature interval up to 300 K. These findings underscores new physics driven by a complex interplay between orbital, spin and lattice degrees of freedom.

cond-mat.str-el↗

arXiv2012

Electronic Origin of High Temperature Superconductivity in Single-Layer FeSe Superconductor

The latest discovery of high temperature superconductivity signature in single-layer FeSe is significant because it is possible to break the superconducting critical temperature ceiling (maximum Tc~55 K) that has been stagnant since the discovery of Fe-based superconductivity in 2008. It also blows the superconductivity community by surprise because such a high Tc is unexpected in FeSe system with the bulk FeSe exhibiting a Tc at only 8 K at ambient pressure which can be enhanced to 38 K under high pressure. The Tc is still unusually high even considering the newly-discovered intercalated FeSe system A_xFe_{2-y}Se_2 (A=K, Cs, Rb and Tl) with a Tc at 32 K at ambient pressure and possible Tc near 48 K under high pressure. Particularly interesting is that such a high temperature superconductivity occurs in a single-layer FeSe system that is considered as a key building block of the Fe-based superconductors. Understanding the origin of high temperature superconductivity in such a strictly two-dimensional FeSe system is crucial to understanding the superconductivity mechanism in Fe-based superconductors in particular, and providing key insights on how to achieve high temperature superconductivity in general. Here we report distinct electronic structure associated with the single-layer FeSe superconductor. Its Fermi surface topology is different from other Fe-based superconductors; it consists only of electron pockets near the zone corner without indication of any Fermi surface around the zone center. Our observation of large and nearly isotropic superconducting gap in this strictly two-dimensional system rules out existence of node in the superconducting gap. These results have provided an unambiguous case that such a unique electronic structure is favorable for realizing high temperature superconductivity.

cond-mat.supr-con↗

arXiv2012

Exotic d-wave superconductivity in strongly hole doped K(x)Ba(1-x)Fe2As2

We investigate the superconducting phase in the K(x)Ba(1-x)Fe2As2 122 compounds from moderate to strong hole-doping regimes. Using functional renormalization group, we show that while the system develops a nodeless anisotropic s+- order parameter in the moderately doped regime, gapping out the electron pockets at strong hole doping drives the system into a nodal cos (kx) cos (ky) d-wave superconducting state. This is in agreement with recent experimental evidence from measurements on KFe2As2 which see a nodal order parameter in the extreme doping regime. The magnetic instability is strongly suppressed. The case of only hole pockets present is argued to be structurally similar to the case of only electron pockets present as indicated for KxFe2Se2.

cond-mat.supr-con↗

arXiv2012

Electronic and Magnetic Structures of Chain Structured Iron Selenide Compounds

Electronic and magnetic structures of iron selenide compounds Ce2O2FeSe2 (2212\ast) and BaFe2Se3(123\ast) are studied by the first-principles calculations. We find that while all these compounds are composed of one-dimensional (1D) Fe chain (or ladder) structures, their electronic structures are not close to be quasi-1D. The magnetic exchange couplings between two nearest-neighbor (NN) chains in 2212\ast and between two NN two-leg-ladders in 123\ast are both antiferromagnetic (AFM), which is consistent with the presence of significant third NN AFM coupling, a common feature shared in other iron-chalcogenides, FeTe (11\ast) and KyFe2-xSe2 (122\ast). In magnetic ground states, each Fe chain of 2212\ast is ferromagnetic and each two-leg ladder of 123\ast form a block-AFM structure. We suggest that all magnetic structures in iron-selenide compounds can be unified into an extended J1-J2-J3 model. Spin-wave excitations of the model are calculated and can be tested by future experiments on these two systems.

cond-mat.str-el↗

arXiv2012

Pressure Effects on Magnetically-Driven Electronic Nematic States in Iron-Pnictides

In a magnetically driven electronic nematic state, an externally applied uniaxial strain rounds the nematic transition and increases the magnetic transition temperature. We study both effects in a simple classical model of the iron-pnictides expressed in terms of local SO(N) spins (with N = 3) which we solve to leading order in 1=N. The magnetic transition temperature is shown to increase linearly in response to an external strain while a sharp crossover, which is a remnant of the nematic transition, can only be identified for extremely small strain. We show that these results can reasonably account for recent neutron experimental data in BaFe2As2 by C. Dhital et al [1].

cond-mat.supr-con↗

arXiv2012

(PI,0) antiferromagnetic spin excitations in superconducting Rb0.82Fe1.68Se2

We use inelastic neutron scattering to show that superconducting (SC) rubidium iron selenide Rb0.82Fe1.68Se2 exhibits antiferromagnetic (AF) spin excitations near the in-plane wave vector Q = (PI, 0) identical to that for iron arsenide superconductors. Moreover, we find that these excitations change from incommensurate to commensurate with increasing energy, and occur at the expense of spin waves associated with the coexisting sqrt(5)\timessqrt(5) block AF phase. Since angle resolved photoemission experiments reveal no evidence for hole-like Fermi surface at Gamma(0, 0), our results suggest that the Q = (PI, 0) excitations in SC Rb0.82Fe1.68Se2 come from localized moments and may have a similar origin as the hourglass-like spin excitations in copper oxide superconductors.

cond-mat.supr-con↗

arXiv2012

Phase diagram as a function of doping level and pressure in Eu$_{1-x}$La$_x$Fe$_2$As$_2$ system

We establish the phase diagram of Eu$_{1-x}$La$_x$Fe$_2$As$_2$ system as a function of doping level x and the pressure by measuring the resistivity and magnetic susceptibility. The pressure can suppress the spin density wave (SDW) and structural transition very efficiently, while enhance the antiferromagnetic (AFM) transition temperature T$_N$ of Eu$^{2+}$. The superconductivity coexists with SDW order at the low pressure, while always coexists with the Eu$^{2+}$ AFM order. The results suggests that Eu$^{2+}$ spin dynamics is disentangeld with superconducting (SC) pairing taken place in the two-dimensional \emph{Fe-As} plane, but it can strongly affect superconducting coherence along c-axis.

cond-mat.supr-con↗

arXiv2011

Nematic orders in Iron-based superconductors

In the newly discovered iron-based superconductors, many experiments have demonstrated the existence of the rotational symmetry breaking nematic order, which has been a prevailing phenomenon in many correlated electronic systems. In this paper, we review nematic behaviors in iron-pnictides and the mechanism behind the development of the nematic order. We discuss evidences that support the spin-driven nematicity in iron-pnictides. Theories, results and predictions will be discussed based on this picture. We also briefly discuss the generalization of this theory to the nematicity in iron-chalcogenides.

cond-mat.str-el↗

arXiv2011

Connectivity of edge and surface states in topological insulators

The edge states of a two-dimensional quantum spin Hall (QSH) insulator form a one-dimensional helical metal which is responsible for the transport property of the QSH insulator. Conceptually, such a one-dimensional helical metal can be attached to any scattering region as the usual metallic leads. We study the analytical property of the scattering matrix for such a conceptual multiterminal scattering problem in the presence of time reversal invariance. As a result, several theorems on the connectivity property of helical edge states in two-dimensional QSH systems as well as surface states of three-dimensional topological insulators are obtained. Without addressing real model details, these theorems, which are phenomenologically obtained, emphasize the general connectivity property of topological edge/surface states from the mere time reversal symmetry restriction.

cond-mat.mes-hall↗

arXiv2011

Neutron Scattering Studies of spin excitations in hole-doped Ba0.67K0.33Fe2As2 superconductor

We report inelastic neutron scattering experiments on single crystals of superconducting Ba0.67K0.33Fe2As2 (Tc = 38 K). In addition to confirming the resonance previously found in powder samples, we find that spin excitations in the normal state form longitudinally elongated ellipses along the QAFM direction in momentum space, consistent with density functional theory predictions. On cooling below Tc, while the resonance preserves its momentum anisotropy as expected, spin excitations at energies below the resonance become essentially isotropic in the in-plane momentum space and dramatically increase their correlation length. These results suggest that the superconducting gap structures in Ba0.67Ka0.33Fe2As2 are more complicated than those suggested from angle resolved photoemission experiments.

cond-mat.supr-con↗

arXiv2011

Inter-layer Superconducting Pairing Induced c-axis Nodal Lines in Iron-based Superconductors

A layered superconductor with a full pairing energy gap can be driven into a nodal superconducting (SC) state by inter-layer pairing when the SC state becomes more quasi-3D. We propose that this mechanism is responsible for the observed nodal behavior in a class of iron-based SCs. We show that the intra- and inter-layer pairings generally compete and the gap nodes develop on one of the {\em hole} Fermi surface pockets as they become larger in the iron-pnictides. Our results provide a natural explanation of the c-axis gap modulations and gap nodes observed by angle resolved photoemission spectroscopy. Moreover, we predict that an anti-correlated c-axis gap modulations on the hole and electron pockets should be observable in the $S^{\pm}$-wave pairing state.

cond-mat.supr-con↗

arXiv2011

Block Antiferromagnetism and Checkerboard Charge Ordering in Alkali-doped Iron Selenides $R$$_{1-x}$Fe$_{2-y}$Se$_2$

By performing first-principles electronic structure calculations and analyzing effective magnetic model of alkali-doped iron selenides, we show that the materials without iron vacancies should approach a novel checkerboard phase in which each four Fe sites group together in tetragonal structure. The checkerboard phase is the ground state with a block antiferromagnetic (AFM) order and a small charge density wave order in the absence of superconductivity. Both of them can also coexist with superconductivity. The results explain mysterious $2\times 2$ ordered patterns and hidden orders observed in various different experiments, clarify the missing link between AFM and superconducting phases, suggest that the block-AFM state is the parent state, and unify the understanding of various observed phases in alkali-doped iron selenides.

cond-mat.supr-con↗