arXiv subjects

Jiangping Hu

Publications and source records attributed to Jiangping Hu.

At least 289 records · Page 16Linked to original sources

arXiv2008

Low energy spin waves and magnetic interactions in SrFe$_2$As$_2$

We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe$_{2}$As$_{2}$ ($T_{N}=220$ K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7 K), the spectrum of magnetic excitations $S(Q,\hbar ω)$ consists of a Bragg peak at the elastic position ($\hbar ω=0$ meV), a spin gap ($ Δ\leq 6.5$ meV), and sharp spin wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across $T_{N} $, the low temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe$_{2}$As$_{2}$ is therefore consistent with a first order phase transition, similar to the structural lattice distortion.

cond-mat.supr-con↗

arXiv2008

Spin and Lattice Structure of Single Crystal SrFe2As2

We use neutron scattering to study the spin and lattice structure on single crystals of SrFe2As2, the parent compound of the FeAs based superconductor (Sr,K)Fe2As2. We find that SrFe2As2 exhibits an abrupt structural phase transitions at 220K, where the structure changes from tetragonal with lattice parameters c > a = b to orthorhombic with c > a > b. At almost the same temperature, Fe spins in SrFe2As2 develop a collinear antiferromagnetic structure along the orthorhombic a-axis with spin direction parallel to this a-axis. These results are consistent with earlier work on the RFeAsO (R = rare earth elements) families of materials and on BaFe2As2, and therefore suggest that static antiferromagnetic order is ubiquitous for the parent compound of these FeAs-based high-transition temperature superconductors.

cond-mat.str-el↗

arXiv2008

Complementary Pair Density Wave and d-wave Checkerboard Order in High Temperature Superconductors

The competing orders in the particle-particle (P-P) channel and the particle-hole (P-H) channel have been proposed separately to explain the pseudogap physics in cuprates. By solving the Bogoliubov-deGennes equation self-consistently, we show that there is a general complementary connection between the d-wave checkerboard order (DWCB) in the particle-hole (P-H) channel and the pair density wave order (PDW) in the particle-particle (P-P) channel. A small pair density localization generates DWCB and PDW orders simultaneously. The result suggests that suppressing superconductivity locally or globally through phase fluctuation should induce both orders in underdoped cuprates. The presence of both DWCB and PDW orders with $4a \times 4a$ periodicity can explain the checkerboard modulation observed in FT-STS from STM and the puzzling dichotomy between the nodal and antinodal regions as well as the characteristic features such as non-dispersive Fermi arc in the pseudogap state.

cond-mat.supr-con↗

arXiv2008

Pairing Symmetry in a Two-Orbital Exchange Coupling Model of Oxypnictides

We study the pairing symmetry of a two orbital $J_1-J_2$ model for FeAs layers in oxypnictides. We vary the doping and the value of $J_1$ and $J_2$ to compare all possible pairing symmetries in a mean-field calculation. We show that the mixture of an intra-orbital unconventional $s_{x^2y^2}\sim \cos(k_x)\cos(k_y)$ pairing symmetry and a $d_{x^2-y^2}\sim \cos(k_x)-\cos(k_y)$ pairing symmetry is favored in a large part of $J_1-J_2$ phase diagram. A pure $ s_{x^2y^2}$ pairing state is favored for $J_2>>J_1$. The signs of the $d_{x^2-y^2}$ order parameters in two different orbitals are opposite. While a small $d_{xy}\sim \sin(k_x)\sin(k_y)$ inter-orbital pairing order coexists in the above phases, the intra-orbital $d_{xy}$ pairing symmetry is not favored even for large values of $J_2$.

cond-mat.supr-con↗

arXiv2008

The magnetoelectric coupling in the multiferroic compound LiCu2O2

We investigate the possible types of coupling between ferroelectricity and magnetism for the zig-zag spin chain multiferroic LiCu2O2 compound. We construct a multi-order parameter phenomenological model for the material based on a group theoretical analysis. From our calculation we conclude that a coupling involving the inter-chain magnetic structure and ferroelectricity is necessary to understand the experimental results of Park et.al. Our proposed model is able to account for the electric polarization flop in the presence of an externally applied magnetic field. Furthermore, based on our theoretical model we can make specific selection rule predictions about electromagnon excitations present in the LiCu2O2 system. We also predict that the electromagnon peaks measured in an ac-conductivity measurement are field dependent.

cond-mat.str-el↗

arXiv2007

Majorana Fermions, Exact Mappings between Classical and Topological Orders

Motivated by the duality between site-centered spin and bond-centered spin in one-dimensional system, which connects two different constructions of fermions from the same set of Majorana fermions, we show that two-dimensional models with topological orders can be constructed from certain well-known models with classical orders characterized by symmetry-breaking. Topology-dependent ground state degeneracy, vanishing two-point correlation functions, and unpaired Majorana fermions on boundaries emerge naturally from such construction. The approach opens a new way to construct and characterize topological orders.

cond-mat.str-el↗

arXiv2007

Microscopic origin of Magnetic Ferroelectrics in Nonlinear Multiferroics

A simple but general microscopic mechanism to understand the interplay between the electric and magnetic degrees of freedom is developed. Within this mechanism, the magnetic structure generates an electric current which induce an counterbalance electric current from the spin orbital coupling. When the magnetic structure is described by a single order parameter, the electric polarization is determined by the single spin orbital coupling parameter, and the material is predicted to be a half insulator. This mechanism provides a simple estimation of the value of ferroelectricity and sets a physical limitation as well.

cond-mat.str-el↗

arXiv2007

Diffusive-Ballistic Crossover and the Persistent Spin Helix

Conventional transport theory focuses on either the diffusive or ballistic regimes and neglects the crossover region between the two. In the presence of spin-orbit coupling, the transport equations are known only in the diffusive regime, where the spin precession angle is small. In this paper, we develop a semiclassical theory of transport valid throughout the diffusive - ballistic crossover of a special SU(2) symmetric spin-orbit coupled system. The theory is also valid in the physically interesting regime where the spin precession angle is large. We obtain exact expressions for the density and spin structure factors in both 2 and 3 dimensional samples with spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2007

d-Wave Checkerboard Order in Cuprates

We show that the d-wave ordering in particle-hole channels, dubbed d-wave checkerboard order, possesses important physics that can sufficiently explain the scanning tunneling microscopy (STM) results in cuprates. A weak d-wave checkerboard order can effectively suppress the coherence peak in the single-particle spectrum while leaving the spectrum along the nodal direction almost unaffected. Simultaneously, it generates a Fermi arc with little dispersion around the nodal points at finite temperature that is consistent with the results of angle-resolved photoemission spectroscopy (ARPES) experiments in the pseudogap phase. We also show that there is a general complementary connection between the d-wave checkerboard order and the pair-density-wave order. Suppressing superconductivity locally or globally through phase fluctuations should induce both orders in underdoped cuprates and explain the nodal-antinodal dichotomy observed in ARPES and STM experiments.

cond-mat.supr-con↗

arXiv2007

Josephson Diode

We propose a new type of Josephson junction formed by two superconductors close to the superconductor-Mott-insulator transition, one of which is doped with holes and the other is doped with electrons. A self-organized Mott-insulating depletion region is formed at the interface between two superconductors, giving rise to an asymmetric response of current to the external voltage. The collective excitations of the depletion region result in a novel phase dynamics that can be measured experimentally in the noise spectrum of the Josephson current.

cond-mat.str-el↗

arXiv2007

The Luttinger liquid kink

Although the Tomonaga-Luttinger liquid exhibits spin-charge separation and the electron is not an elementary excitation of the system, nevertheless an effective electronic dispersion may be defined by the frequency-dependent peak in the momentum distribution curve (MDC). The MDC is defined by considering the single hole spectral function $A^<(\vec{k},ω)$ as a function of $\vec{k}$ at a fixed frequency $ω$. We show the existence of a \emph{kink} in this dispersion for the spin-rotationally invariant Tomonaga-Luttinger liquid at finite temperature. In the repulsive regime where the charge velocity $v_{c}$ is greater than the spin velocity $v_{s}$, the low frequency effective electronic dispersion is linear in $\vec{k}$ and follows a velocity $v_l$ between the spin and charge velocities, $v_s < v_l < v_c$. The high frequency part (which is also linear in $\vec{k}$) disperses with the charge velocity $v_c$. The energy scale of the crossover between the two velocities defines a kink, $E_{\rm kink}$. In addition, the high energy dispersion extrapolates to the Fermi energy at a wavevector $k_{ex} \ne k_F$ which is shifted from the Fermi wavevector. The presence of such a kink is measurable with, {\em e.g.}, angle-resolved photoemission experiments, and may be used to test for the presence of Luttinger liquid behavior in systems in which the separate contributions from spin and charge to the MDC are not discernible due to strong interactions or low experimental resolution.

cond-mat.str-el↗

arXiv2007

Tracking control for multi-agent consensus with an active leader and variable topology

In this paper, we consider the coordination control of a group of autonomous mobile agents with multiple leaders. Different interconnection topologies are investigated. At first, a necessary and sufficient condition is proved in the case of fixed interconnection topology. Then a sufficient condition is proposed when the interconnection topology is switched. With a simple first-order dynamics model by using the neighborhood rule, both results show that the group behavior of the agents will converge to the polytope formed by the leaders.

math-ph↗

arXiv2007

Leader-following coordination of multi-agent systems with coupling time delays

In this paper, we consider a leader-following consensus problem of a group of autonomous agents with time-varying coupling delays. Two different cases of coupling topologies are investigated. At first, a necessary and sufficient condition is proved in the case when the interconnection topology is fixed and directed. Then a sufficient condition is proposed in the case when the coupling topology is switched and balanced. Numerical examples are also given to illustrate our results.

math-ph↗

arXiv2007

Coordination for a Group of Autonomous Mobile Agents with Multiple Leaders

math-ph↗

arXiv2007

An Effective Model of Magnetoelectricity in Multiferroics $RMn_2O_5$

An effective model is developed to explain the phase diagram and the mechanism of magnetoelectric coupling in multiferroics, $RMn_2O_5$. We show that the nature of magnetoelectric coupling in $RMn_2O_5$ is a coupling between two Ising-type orders, namely, the ferroelectric order in the b axis, and the coupled magnetic order between two frustrated antiferromagnetic chains. The frustrated magnetic structure drives the system to a commensurate-incommensurate phase transition, which can be understood as a competition between a collinear or col-plane order stemming from the `order by disorder' mechanism and a chiral symmetry order. The low energy excitation is calculated and the effect of the external magnetic field is analyzed. Distinct features in the electromagnon spectrums in the incommensurate phase are predicted.

cond-mat.str-el↗

arXiv2006

Nature of the Quantum Phase Transition in Quantum Compass Model

In this work, we show that the quantum compass model on an square lattice can be mapped to a fermionic model with local density interaction. We introduce a mean-field approximation where the most important fluctuations, those perpendicular to the ordering direction, are taken into account exactly. It is found that the quantum phase transition point at $J_x=J_z$ marks a first order phase transition. We also show that the mean field result is robust against the remaining fluctuation corrections up to the second order.

cond-mat.str-el↗

arXiv2006

A Magnetic Model of the Tetragonal-Orthorhombic Transition in the Cuprates

It is shown that a quasi two dimensional (layered) Heisenberg antiferromagnet with fully frustrated interplane couplings ({\it e.g.} on a body-centered tetragonal lattice) generically exhibits two thermal phase transitions with lowering temperature -- an upper transition at $T_{TO}$ (``order from disorder without order'') in which the lattice point-group symmetry is spontaneously broken, and a lower Néel transition at $T_{N}$ at which spin-rotation symmetry is broken. Although this is the same sequence of transitions observed in La$_2$CuO$_4$, in the Heisenberg model (without additional lattice degrees of freedom) $(T_{TO}-T_N) /T_N$ is much smaller than is observed. The model may apply to the bilayer cuprate La$_2$CaCuO$_6$, in which the transitions are nearly coincident.

cond-mat.str-el↗

arXiv2005

Topological Orbital Angular Momentum Hall Current

We show that there is a fundamental difference between spin Hall current and orbital angular momentum Hall current in Rashba- Dresselhaus spin orbit coupling systems. The orbital angular momentum Hall current has a pure topological contribution which is originated from the existence of magnetic flux in momentum space while there is no such topological nature for the spin Hall current. Moreover, we show that the orbital Hall conductance is always larger than the spin Hall conductance in the presence of both couplings. The topological part is expected to be free from the effect of disorder due to the topological nature. Therefore, the orbital angular momentum Hall current should be the major effect in real experiments.

cond-mat.mes-hall↗