arXiv subjects

Jiangping Hu

Publications and source records attributed to Jiangping Hu.

At least 271 records · Page 15Linked to original sources

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

Quintet pairing and non-Abelian vortex string in spin-3/2 cold atomic systems

We study the s-wave quintet Cooper pairing phase ($S_{tot}=2$) in spin-3/2 cold atomic systems and identify various novel features which do not appear in the spin-1/2 counterpart. A single quantum vortex is shown to be energetically less stable than a pair of half-quantum vortices (HQV). The HQV exhibits the global analogue of the non-Abelian Alice string and SO(4) Cheshire charge in gauge theories. The non-Abelian HQV loop enables topological generation of quantum entanglement.

cond-mat.str-el↗

arXiv2009

Quantized Quasi-Two Dimensional Bose-Einstein Condensates with Spatially Modulated Nonlinearity

We investigate the localized nonlinear matter waves of the quasi-two dimensional Bose-Einstein condensates with spatially modulated nonlinearity in harmonic potential. It is shown that the whole Bose-Einstein condensates, similar to the linear harmonic oscillator, can have an arbitrary number of localized nonlinear matter waves with discrete energies, which are mathematically exact orthogonal solutions of the Gross-Pitaevskii equation. Their novel properties are determined by the principle quantum number n and secondary quantum number l: the parity of the matter wave functions and the corresponding energy levels depend only on n, and the numbers of density packets for each quantum state depend on both n and l which describe the topological properties of the atom packets. We also give an experimental protocol to observe these novel phenomena in future experiments.

cond-mat.quant-gas↗

arXiv2009

Quasiparticle scattering in two dimensional helical liquid

We study the quasiparticle interference (QPI) patterns caused by scattering off nonmagnetic, magnetic point impurities, and edge impurities, separately, in a two dimensional helical liquid, which describes the surface states of a topological insulator. The unique features associated with hexagonal warping effects are identified in the QPI patterns of charge density with nonmagnetic impurities and spin density with magnetic impurities. The symmetry properties of the QPI patterns can be used to determine the symmetry of microscopic models. The Friedel oscillation is calculated for edge impurities and the decay of the oscillation is not universal, strongly depending on Fermi energy. Some discrepancies between our theoretical results and current experimental observations are discussed.

cond-mat.mes-hall↗

arXiv2009

Evidence of Magnetically Driven Structural Phase Transition in Parent Compounds RFeAsO (R = La, Sm, Gd, Tb): study of low-temperature X-ray diffraction

We report measurements of structural phase transition of four parent compounds $R$FeAsO ($R$ = La, Sm, Gd, and Tb) by means of low-temperature X-ray diffraction (XRD). Magnetic transition temperatures associated with Fe ions ($T_{N1}$) are also determined from the temperature dependence of resistivity. As $R$ is changed from La, through Sm and Gd, to Tb, both the c-axis and a-axis lattice constants decrease significantly. Meanwhile both the structural phase transition temperature ($T_S$) and $T_{N1}$ decrease monotonously. It is also found that the temperature gap between $T_S$ and $T_{N1}$ becomes smaller when the distance between FeAs layer becomes shorter. This result is consistent with magnetically driven structural phase transition and suggests that the dimensionality have an important effect on the AFM ordering.

cond-mat.supr-con↗

arXiv2009

Impurity-induced bound states in iron-based superconductors with s-wave cos(kx)cos(ky) pairing symmetry

Using both the self-consistent Bogoliubov-de Gennes formulation and non-self-consistent T-matrix approach, we perform a comprehensive investigation of the in-gap bound states induced by a localized single impurity in iron-based superconductors. We focus on studying signatures associated with the unconventional sign-changed s-wave pairing symmetry. For a non-magnetic impurity, we find that there are two in-gap bounds, symmetric with respect to zero energy, only in the sign changed s-wave pairing state, not in the sign-unchanged s-wave state, due to the existence of non-trivial Andreev bound states caused by the sign change. For a magnetic impurity, we find that due to the breakdown of the local time-reversal symmetry, there exist only bound state solutions (with orbital degeneracy) carrying one of the electron-spin polarizations around the impurity. As increasing the scattering strength, the system undergoes a quantum phase transition (level crossing) from a spin-unpolarized ground state to a spin-polarized one. While the results for the magnetic impurity are qualitatively similar in both the sign-changed and sign-unchanged s-wave superconducting states, the bound states in the first case are more robust and there is no $π$ phase shift of the SC gap near the impurity in the strong scattering regime.

cond-mat.supr-con↗

arXiv2009

Doping-dependent pairing symmetry in the Iron-Pnictides

We use the functional renormalization group method to analyze the phase diagram of a 4-band model for the iron-pnictides subject to band interactions with certain A_{1g} momentum dependence. We determine the parameter regimes where an extended s-wave pairing instability with and without nodes emerges. On the electron-doped side, the parameter regime in which a nodal gap appears is found to be much narrower than recently predicted in arXiv:0903.5547. On the hole-doped side, the extended s-wave pairing never becomes nodal: above a critical strength of the intra-band repulsion, the system favors an exotic extended d-wave instability on the enlarged hole pockets at much lower T_c. At half filling, we find that a strong momentum dependence of inter-band pair hopping yields an extended s-wave instability instead of spin-density wave (SDW) ordering. These results demonstrate that an interaction anisotropy around the Fermi surfaces generally leads to a pronounced sensitivity of the pairing state on the system parameters.

cond-mat.supr-con↗

arXiv2009

Effect of electron-doping on spin excitations of underdoped BaFe$_{1.96}$Ni$_{0.04}$As$_{2}$

We use neutron scattering to study magnetic order and spin excitations in BaFe$_{1.96}$Ni$_{0.04}$As$_{2}$. On cooling, the system first changes the lattice symmetry from tetragonal to orthoromhbic near $\sim$97 K, and then orders antiferromagnetically at $T_N=91$ K before developing weak superconductivity below $\sim$15 K. Although superconductivity appears to co-exist with static antiferromagnetic order from transport and neutron diffraction measurement, inelastic neutron scattering experiments reveal that magnetic excitations do not respond to superconductivity. Instead, the effect of electron-doping is to reduce the c-axis exchange coupling in BaFe$_2$As$_2$ and induce quasi two-dimensional spin excitations. These results suggest that transition from three-dimensional spin waves to two-dimensional spin excitations by electron-doping is important for the separated structural/magnetic phase transitions and high-temperature superconductivity in iron arsenides.

cond-mat.supr-con↗

arXiv2009

Field theory for magnetic and lattice structure properties of Fe_{1+y}Te_{1-x}Se_x

We study the magnetism and lattice distortion of Fe_{1+y}Te_{1-x}Se_x with a general field theory formalism, motivated by recent neutron scattering experiments. Besides the Ising nematic order parameter which can be naturally defined in the 1111 and 122 materials, We show that the collinear order observed in $\mathrm{Fe_{1+y}Te}$ materials is stabilized by an extra Ising order parameter which doubles the generalized unit cell of the system. By tuning this Ising order parameter, one can drive a transition between collinear and noncollinear spin density wave orders, both of which are observed in the FeTe family. The nature of the quantum and classical phase transitions are also studied in this field theory formalism, with lattice strain tensor fluctuation considered.

cond-mat.str-el↗

arXiv2009

Spin Waves and Magnetic Exchange Interactions in CaFe2As2

Antiferromagnetism is relevant to high temperature (high-Tc) superconductivity because copper oxide and iron arsenide high-Tc superconductors arise from electron- or hole-doping of their antiferromagnetic (AF) ordered parent compounds. There are two broad classes of explanation for the phenomenon of antiferromagnetism: in the local moment picture, appropriate for the insulating copper oxides, AF interactions are well described by a Heisenberg Hamiltonian; while in the itinerant model, suitable for metallic chromium, AF order arises from quasiparticle excitations of a nested Fermi surface. There has been contradictory evidence regarding the microscopic origin of the AF order in iron arsenide materials, with some favoring a localized picture while others supporting an itinerant point of view. More importantly, there has not even been agreement about the simplest effective ground state Hamiltonian necessary to describe the AF order. Here we report inelastic neutron scattering mapping of spin-wave excitations in CaFe2As2, a parent compound of the iron arsenide family of superconductors. We find that the spin waves in the entire Brillouin zone can be described by an effective three-dimensional local moment Heisenberg Hamiltonian, but the large in-plane anisotropy cannot. Therefore, magnetism in the parent compounds of iron arsenide superconductors is neither purely local nor purely itinerant; rather it is a complicated mix of the two.

cond-mat.supr-con↗

arXiv2009

Magnetic Properties of the Superconducting State of Iron-Based Superconductors

We show that features of the dynamical spin susceptibility can unambiguously distinguish between different pairing symmetries of the superconducting state in iron-based superconductors. A magnetic resonance is a unique feature for the extended $s_{x^2y^2}$-wave $\cos k_x\cos k_y$ pairing symmetry. It is present in the pure superconducting (SC) state, but weakens in the mixed SC and magnetically ordered state. We calculate the the RPA correction to the NMR spin relaxation rate $1/T_1$ and the Knight shift in the above states and show a good agreement with experimental results. Moreover, we argue that the energy dispersion of the magnetic resonance along c axis observed in neutron scattering experiments is also an indirect evidence supporting the $s_{x^2y^2}$ pairing symmetry.

cond-mat.supr-con↗

arXiv2009

Quasiparticle Scattering Interference in Superconducting Iron-pnictides

Using both two orbital and five orbital models, we investigate the quasiparticle interference (QPI) patterns in the superconducting (SC) state of iron-based superconductors. We compare the results for nonmagnetic and magnetic impurities in sign-changed s-wave $\cos(k_x)\cdot\cos(k_y)$ and sign-unchanged $|\cos(k_x)\cdot\cos(k_y)|$ SC states. While the patterns strongly depend on the chosen band structures, the sensitivity of peaks around $(\pmπ,0)$ and $(0,\pmπ)$ wavevectors on magnetic or non-magnetic impurity, and sign change or sign unchanged SC orders is common in two models. Our results strongly suggest that QPI may provide direct information of band structures and evidence of the pairing symmetry in the SC states.

cond-mat.supr-con↗

arXiv2008

Three-dimensional Resonance in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$

We use inelastic neutron scattering to study magnetic excitations of the FeAs-based superconductor BaFe$_{1.9}$Ni$_{0.1}$As$_2$ above and below its superconducting transition temperature $T_c=20$ K. In addition to gradually open a spin gap at the in-plane antiferromagnetic ordering wavevector $(1,0,0)$, the effect of superconductivity is to form a three dimensional resonance with clear dispersion along the c-axis direction. The intensity of the resonance develops like a superconducting order parameter, and the mode occurs at distinctively different energies at $(1,0,0)$ and $(1,0,1)$. If the resonance energy is directly associated with the superconducting gap energy $Δ$, then $Δ$ is dependent on the wavevector transfers along the c-axis. These results suggest that one must be careful in interpreting the superconducting gap energies obtained by surface sensitive probes such as scanning tunneling microscopy and angle resolved photoemission.

cond-mat.str-el↗

arXiv2008

Theory of Magnetic Order in $ Fe_{1+y}Te_{1-x}Se_x$

We develop a local spin model to explain the rich magnetic structures in the iron-based superconductors $Fe_{1+y}Te_{1-x}Se_x$. We show that our model exhibits both commensurate antiferromagnetic and incommensurate magnetic order along the crystal a-axis. The transition from the commensurate to the incommensurate phase is induced when the concentration of excess $Fe$ atoms is larger than a critical value. Experimentally measurable spin-wave features are calculated, and the mean-field phase diagram of the model is obtained. Our model also suggests the existence of a large quantum critical region due to strong spin frustration upon increasing $Se$ concentration.

cond-mat.supr-con↗

arXiv2008

First-order magnetic and structural phase transitions in Fe$_{1+y}$Se$_x$Te$_{1-x}$

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe$_{1+y}$Se% $_x$Te$_{1-x}$. Fe$_{1.068}$Te exhibits a first order phase transition near 67 K with a tetragonal to monoclinic structural transition and simultaneously develops a collinear antiferromagnetic (AF) order responsible for the entropy change across the transition. Systematic studies of FeSe$%_{1-x}$Te$_x$ system reveal that the AF structure and lattice distortion in these materials are different from those of FeAs-based pnictides. These results call into question the conclusions of present density functional calculations, where FeSe$_{1-x}$Te$_x$ and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density-wave AF order.

cond-mat.supr-con↗

arXiv2008

Localization and Kosterlitz-Thouless Transition in Disordered Graphene

We investigate disordered graphene with strong long-range impurities. Contrary to the common belief that delocalization should persist in such a system against any disorder, as the system is ex-pected to be equivalent to a disordered two-dimensional Dirac Fermionic system, we find that states near the Dirac points are localized for sufficiently strong disorder and the transition between the localized and delocalized states is of Kosterlitz-Thouless type. Our results show that the transition originates from bounding and unbounding of local current vortices.

cond-mat.dis-nn↗

arXiv2008

Bi-collinear antiferromagnetic order in the tetragonal $α$-FeTe

By the first-principles electronic structure calculations, we find that the ground state of PbO-type tetragonal $α$-FeTe is in a bi-collinear antiferromagnetic state, in which the Fe local moments ($\sim2.5μ_B$) are ordered ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe square lattice. This bi-collinear order results from the interplay among the nearest, next nearest, and next next nearest neighbor superexchange interactions $J_1$, $J_2$, and $J_3$, mediated by Te $5p$-band. In contrast, the ground state of $α$-FeSe is in the collinear antiferromagnetic order, similar as in LaFeAsO and BaFe$_2$As$_2$.

cond-mat.mtrl-sci↗

arXiv2008

Experimental Consequences of the S-wave $\cos(k_x) \cdot \cos(k_y)$ Superconductivity in the Iron-Pnictides

The experimental consequences of different order parameters in iron-based superconductors are theoretically analyzed. We consider both nodeless and nodal order parameters, with an emphasis on the $\cos(k_x)\cdot \cos(k_y)$ nodeless order parameter recently derived by two of us. We analyze the effect of this order parameter on the spectral function, density of states, tunneling differential conductance, penetration depth, and the NMR spin relaxation time. This extended s-wave symmetry has line-zeroes in between the electron and hole pockets, but they do not intersect the two Fermi surfaces for moderate doping, and the superconductor is fully gapped. However, this suggests several quantitative tests: the exponential decay of the penetration depth weakens and the density of states reveals a smaller gap upon electron or hole doping. Moreover, the $\cos(k_x) \cdot \cos(k_y)$ superconducting gap is largest on the smallest (hole) Fermi surface. For the $1/T_1$ NMR spin relaxation rate, the inter-band contribution is consistent with the current experimental results, including a (non-universal) $T^{3}$ behavior and the absence of a coherence peak. However, the intra-band contribution is considerably larger than the inter-band contributions and still exhibits a small enhancement in the NMR spin relaxation rate right below $T_c$ in the clean limit.

cond-mat.supr-con↗