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Xiao Hu

Publications and source records attributed to Xiao Hu.

At least 271 records · Page 15Linked to original sources
arXiv2008

Possible Dynamic States in Inductively Coupled Intrinsic Josephson Junctions of Layered High-$T_c$ Superconductors

Based on computer simulations and theoretical analysis, a new dynamic state is found in inductively coupled intrinsic Josephson junctions in the absence of an external magnetic field. In this state, the plasma oscillation is uniform along the c axis and there are $(2m+1)π$ phase kinks, with $m$ being an integer, periodic and thus non-uniform in the $c$ direction. In the IV characteristics, the state manifests itself as current steps occurring at all cavity modes. Inside the current steps, the plasma oscillation becomes strong, which generates several harmonics in frequency spectra at a given voltage. The recent experiments on terahertz radiations from the mesa of a BSCCO single crystal can be explained in terms of this state.

cond-mat.supr-con↗
arXiv2007

Computer Simulation on Terahertz Emission from Intrinsic Josephson Junctions of High-T_c Superconductors

Solving coupled nonlinear sine-Gordon equations and Maxwell equations numerically, we study the electromagnetic and superconducting properties of the single crystal of high-$T_c$ superconductor $\rm{Bi_2Sr_2CaCu_2O_{8+δ}}$ with a static magnetic field applied parallel to the $ab$-plane and a dc current fed in along the c-axis. Cavity resonances of transverse plasma occur in the intrinsic Josephson junctions with frequencies in terahertz regime. It is revealed that the electromagnetic wave can transmit from the junctions into space. The emitted energy counted by the Poynting vector is about $400\rm{W/cm^2}$. The frequency as well as the energy of emission can be tuned almost continuously by the current and magnetic field.

cond-mat.supr-con↗
arXiv2006

Depinning and creep motion in glass states of flux lines

Using dynamical computer simulation we have investigated vortex matters in glass states. A genuine continuous depinning transition is observed at zero temperature, which also governs the low-temperature creep motion. With the notion of scaling, we evaluate in high accuracy critical exponents and scaling functions; we observe a non-Arrhenius creep motion for weak collective pinning where Bragg glass (BrG) is stabilized at equilibrium, while for strong pinning the well-known Arrhenius law is recovered. In both cases, a sharp crossover takes place between depinning and creep at low temperatures. The possible relation between the present results and a recent experimental observation of a second-order like phase boundary inside the BrG phase is discussed.

cond-mat.supr-con↗
arXiv2006

Effective field theory with a $θ$-vacua structure for 2d spin systems

We construct a nonlinear sigma (NL$σ$) model description of 2+1d spin systems, by coupling together antiferromagnetic spin chains via interchain exchange terms. Our mapping incorporates methods developed recently by ourselves and by Senthil and Fisher, which aim at describing competition between antiferromagnetic and valence-bond-solid orders in quantum magnets. The resulting 2+1d $O$(4) NL$σ$ model contains a topological $θ$-term whose vacuum angle $θ$ varies continuously with $δ$, the bond-alternation strength of the interchain interaction. This implies that the $θ$-vacua structure for this NL$σ$ model can be explored by tuning $δ$ in a suitable 2d spin system, which is strongly reminiscent of the situation for 1+1 AF spin chains with bond-alternation.

cond-mat.str-el↗
arXiv2006

Energy landscape and shear modulus of interlayer Josephson vortex systems

The ground state of interlayer Josephson vortex systems is investigated on the basis of a simplified Lawrence-Doniach model in which spatial dependence of the gauge field and the amplitude of superconducting order parameter is not taken into account. Energy landscape is drawn with respect to the in-plane field, the period of insulating layers including Josephson vortices, and the shift from the aligned vortex lattice. The energy landscape has a multi-valley structure and ground-state configurations correspond to bifurcation points of the valleys. In the high-field region, the shear modulus becomes independent of field and its anisotropy dependence is given by $c_{66}\propto γ^{-4}$.

cond-mat.supr-con↗
arXiv2006

Magnetic and Electronic Properties of the New Ferrimagnet Sr8CaRe3Cu4O24

Magnetic and electronic properties of the recently-discovered material Sr8CaRe3Cu4O24 were investigated by means of a quantum Monte Carlo simulation, the Green function method and the LSDA+U (local spin-density approximation plus the Hubbard-U term) method. The LSDA+U calculation shows that the ground state is an insulator with magnetic moment M=1.01\muB/f.u., which is consistent with experimental results. The magnetic sites were specified and an effective model for the magnetic properties of this compound derived. The resultant effective model is a three-dimensional Heisenberg model with spin-alternation. Finite-temperature properties of this effective model are investigated by the quantum Monte Carlo method (continuous-time loop algorithm) and the Green function method. The numerical results are consistent with experimental results, indicating that the model is suitable for this material. Using the analysis of the effective model, some predictions for the material are made.

cond-mat.str-el↗
arXiv2006

Anomalous impurity effect on magnetization in frustrated one-dimensional ferro- and ferrimagnets

Significant decrease of spontaneous magnetization in frustrated one-dimensional ferro- and ferrimagnets due to non-magnetic impurities is predicted. Using the density-matrix renormalization group method and the exact diagonalization method, we confirm that the total spin can vanish due to a single impurity in finite chains. Introducing the picture of magnetic domain inversion, we numerically investigate the impurity-density dependence of magnetization. In particular, we show that even with an infinitesimal density of impurities the magnetization in the ground state is reduced by about 40% from that of the corresponding pure system. Conditions for the materials which may show this anomalous impurity effect are formulated.

cond-mat.str-el↗
arXiv2005

Coexistence of a nearly spin-singlet state and antiferromagnetic long-range order in quantum spin system Cu$_2$CdB$_2$O$_6$

A 1/2 magnetization plateau in magnetic fields above 23 T and antiferromagnetic (AF) long-range order (AFLRO) in low fields were found in Cu$_2$CdB$_2$O$_6$. Experimental results agree with quantum Monte Carlo results for an expected spin system. There are two kinds of Cu sites [Cu(1) and Cu(2)], which are located adjacent to each other. Unexpectedly, spins on the Cu(1) and Cu(2) sites are in a nearly spin-singlet state and form AFLRO, respectively, although interactions between the Cu(1) and Cu(2) spins cannot be ignored. Cu$_2$CdB$_2$O$_6$ is the first material which shows such coexistence in an atomic scale.

cond-mat.str-el↗
arXiv2005

Robust Half-Metallic Character and Large Oxygen Magnetism in a Perovskite Cuprate

The new perovskite cuprate material Sr$_{8}$CaRe$_{3}$Cu$_{4}$O$_{24}$, which behaves ferrimagnetically and shows an unusually high Curie temperature ($T_c \sim$ 440 K), is found from density-functional theory calculation to display several surprising properties after hole doping or chemical substitution: (1) Half metal (HM) is realized by replacing Re with W or Mo while $T_c$ remains high; (2) hole-doped Sr$_{8}$CaRe$_{3}$Cu$_{4}$O$_{24}$ is also HM with high $T_c$. Moreover, we find that the O atoms will carry a large magnetic moment after hole doping, which is in sharp contrast with the generally accepted concept that magnetism in solid requires partially filled shells of $d$ or $f$ electrons in cations. The material Sr$_8$CaRe$_3$Cu$_4$O$_{24}$ is therefore expected to provide a very useful platform for material design and development.

cond-mat.mtrl-sci↗
arXiv2005

Competition between Hidden Spin and Charge Orderings in Stripe Phase

The correlation between charge and spin orderings in hole-doped antiferromagnets is studied within an effective model of quantum strings fluctuating in an antiferromagnetic background. In particular, we perform the direct estimation of the charge and spin long-range-order parameters by means of the quantum Monte Carlo simulation. A hidden spin long-range order is found to be governed by a competition between the two trends caused by increasing hole mobility: the enhancement of the two-dimensional spin-spin correlation mediated by hole motions and the reformation of a strong stripe order.

cond-mat.str-el↗
arXiv2004

Reply to "Comment on "Effects of Point Defects on the Phase Diagram of Vortex States in High-Tc Superconductors in the B//c Axis""

We reply to a recent Comment on our previous article [Phys. Rev. Lett. 86, 5140 (2001)] by Olsson and Teitel [cond-mat/0404473]. We point out that their scaling argument to question the possible vortex slush phase in the frustrated XY model with point defects cannot be sufficiently justified by the provided numerical data, and that their data are indeed consistent with our previous article.

cond-mat.supr-con↗
arXiv2004

Density functional theory for freezing transition of vortex-line liquid with periodic layer pinning

By the density functional theory for crystallization, it is shown that for vortex lines in an underlying layered structure a smectic phase with period m=2 can be stabilized by strong layer pinning. The freezing of vortex liquid is then two-step, a second-order liquid-smectic transition and a first-order smectic-lattice transition. DFT also indicates that a direct, first-order liquid-lattice transition preempts the smectic order with m>2 irrespectively of the pinning strength. Possible H-T phase diagrams are mapped out. Implications of the DFT results to the interlayer Josephson vortex system in high-Tc cuprates are given.

cond-mat.supr-con↗
arXiv2004

Decoupled two-dimensional superconductivity and continuous melting transitions in layered superconductors immersed in a parallel magnetic field

Possible phases and the B-T phase diagram of interlayer Josephson vortices induced by a magnetic field parallel to the superconducting layers are investigated by Monte Carlo simulations based on the anisotropic, frustrated XY model. While for low magnetic fields and small anisotropy parameters a single first-order transition is observed similarly to the melting of Abrikosov (or pancake) vortex lattice, an intermediate phase, characterized by decoupled, two-dimensional (2D) quasi long-range crystalline order (QLRCO) and superconductivity, is found at high magnetic fields and large anisotropy parameters. Combining the simulation results with a symmetry argument, it is revealed that this intermediate phase is of Kosterlitz-Thouless (KT) type, and the melting of 2D quasi Josephson vortex lattices and suppression of superconductivity is a KT transition. Evolution of the intermediate phase to the low-temperature phase of 3D LRCO is second order and belongs to the 3D XY universality class. The three phase boundaries merge at a multicritical point. It is revealed that decoupling of the 3D Josephson vortex lattice into the 2D phase is triggered by hops of Josephson flux lines across superconducting layers activated by thermal fluctuations. The equilibrium phase diagram with the KT phase at high magnetic fields and large anisotropy parameters is consistent with the peculiar Lorentz-force-independent dissipation observed in highly anisotropic high-Tc superconductor Bi2Sr2CaCu2O(8+y) by Iye et al. (Physica 159C, 433 (1989)).

cond-mat.supr-con↗
arXiv2004

Orbital order and ferrimagnetic properties of the new compound $Sr_8 Ca Re_3 Cu_4 O_{24}$

By means of the LSDA+U method and the Green function method, we investigate the electronic and magnetic properties of the new material of Sr$_8$CaRe$_3$Cu$_4$O$_{24}$. Our LSDA+U calculation shows that this system is an insulator with a net magnetic moment of 1.01 $μ_{\rm B}$/f.u., which is in good agreement with the experiment. Magnetic moments are mainly located at Cu atoms, and the magnetic moments of neighboring Cu sites align anti-parallel. It is the non-magnetic Re atoms that induce an orbital order of $d$ electrons of Cu atoms, which is responsible for the strong exchange interaction and the high magnetic transition temperature. Based on the LSDA+U results, we introduce an effective model for the spin degrees of freedom, and investigate the finite-temperature properties by the Green function method. The obtained results are consistent with the experimental results, indicating that the spin-alternating Heisenberg model is suitable for this compound.

cond-mat.mtrl-sci↗
arXiv2003

Kosterlitz-Thouless phase and continuous melting transition in layered superconductors immersed in a parallel magnetic field

B-T phase diagram and phase transitions of interlayer Josephson vortices are investigated. For magnetic fields above a critical value, we find a Kosterlitz-Thouless (KT) type intermediate phase characterized by in-plane two-dimensional, quasi long-range orders (QLROs) of vortex alignment and superconductivity. The transformation to high-temperature normal phase is a KT transition. The evolution to low-temperature phase of three-dimensional (3D) LRO of vortex alignment is a second-order phase transition in the 3D XY universality class. For low magnetic fields, a single first-order melting transition of the Josephson vortex lattice is observed. The present study therefore indicates a multicritical point in the B-T phase diagram.

cond-mat.supr-con↗
arXiv2003

Possible spin triplet superconductivity in Na$_x$CoO$_{2}\cdot y$H$_{2}$0

Combining symmetry based considerations with inputs from available experimental results, we make the case that a novel spin-triplet superconductivity triggered by antiferromagnetic fluctuations may be realized in the newly discovered layered cobaltide Na$_x$CoO$_{2}\cdot y$H$_2$O. In the proposed picture, unaccessable via resonating-valence-bond physics extrapolated from half-filling, the pairing process is similar to that advanced for Sr$_{2}$RuO$_4$, but enjoys a further advantage coming from the hexagonal structure of the Fermi-surface which gives a stronger pairing tendency.

cond-mat.str-el↗
arXiv2003

Scalar chiral ground states of spin ladders with four-spin exchanges

We show that scalar chiral order can be induced by four-spin exchanges in the two-leg spin ladder, using the spin-chirality duality transformation and matrix-product ansatz. Scalar-chiral-ordered states are found to be exact ground states in a family of spin ladder models. In this scalar chiral phase, there is a finite energy gap above the doubly degenerate ground states and a $Z_2 \times Z_2 \times Z_2$ symmetry is fully broken. It is also shown that the SU(4)-symmetric model, which is self-dual under the duality transformation, is on a multicritical point surrounded by the staggered dimer phase, the staggered scalar chiral phase, and the gapless phase.

cond-mat.str-el↗