arXiv subjects

Xiao Hu

Publications and source records attributed to Xiao Hu.

At least 253 records · Page 14Linked to original sources

arXiv2010

Interference and Switching of Josephson current carried by nonlocal spin-entangled electrons in a SQUID-like system with quantum dots

Josephson current of spin-entangled electrons through the two branches of a SQUID-like structure with two quantum dots exhibits a magnetic-flux response different from the conventional Josephson current. Due to their interference, the period of maximum Josephson current changes from $h/2e$ to $h/e$, which can be used for detecting the Cooper-pair splitting efficiency. The nonlocal spin entanglement provides a quantum mechanical functionale for switching on and off this novel Josephson current, and explicitly a switch is formulated by including a pilot junction. It is shown that the device can be used to measure the magnitude of split-tunneling Josephson current.

cond-mat.supr-con↗

arXiv2010

Half-metallic Antiferromagnet Sheets in Sr4M2O6CrFeAs2 (M=Sc, Cr) and Their Bulk Form

We reveal by first-principles calculations that in iron pnictides Sr4M2O6CrFeAs2 (M=Sc, Cr) the two-dimensional CrFeAs2 layers exhibit a robust band structure of half-metallic antiferromagnet (HMAFM). Due to the thick blocking layer, the interlayer coupling is vanishingly small and thus the conductive channels in individual layers may take the alternative spin direction randomly. We show that, since the spin magnetizations of Fe and Cr are different in a ferromagnetic state, applying a strong magnetic field and ramping it down gradually can align the spin direction of conductive channel of all HMAFM layers, which restores the bulk HMAFM.

cond-mat.supr-con↗

arXiv2010

Half-metallic Antiferromagnet BaCrFeAs2

First-principles calculations and a tight-binding analysis predict that the iron-pnictide BaCrFeAs2 is a promising candidate for half-metallic material with fully-compensated magnetization. The transition-metal ions Cr and Fe prefer the three-dimensional intervening lattice, which yields the antiferromagnetic order of spin orientations. Due to the difference between Cr and Fe in the electronegativity, a band gap is opened at the Fermi level in the spin channel in which Fe provides the majority carriers. The selective hybridization between 3d orbitals of Cr and As:4p states due to the peculiar lattice structure of the iron-pnictide is shown to be crucial for the novel properties.

cond-mat.mtrl-sci↗

arXiv2010

Response and Amplification of Terahertz Electromagnetic Waves in Intrinsic Josephson Junctions of Layered High-Tc Superconductor

We investigate the response of a stack of intrinsic Josephson junctions (IJJs) to terahertz (THz) electromagnetic (EM) irradiation. A significant amplification of the EM wave can be achieved by the IJJs stack when the incident frequency equals to one of the cavity frequencies. The irradiation excites pi phase kinks in the junctions, which stimulate the cavity resonance when the bias voltage is tuned. A large amount of dc energy is then pumped into the Josephson plasma oscillation, and the incident wave gets amplified. From the profound current step in IV characteristics induced at the cavity resonance, the system can also be used for detection of the THz wave.

cond-mat.supr-con↗

arXiv2010

Local electronic structures on the superconducting interface $LaAlO_{3}/SrTiO_{3}$

Motivated by the recent discovery of superconductivity on the heterointerface $LaAlO_{3}/SrTiO_{3}$, we theoretically investigate its local electronic structures near an impurity considering the influence of Rashba-type spin-orbit interaction (RSOI) originated in the lack of inversion symmetry. We find that local density of states near an impurity exhibits the in-gap resonance peaks due to the quasiparticle scattering on the Fermi surface with the reversal sign of the pairing gap caused by the mixed singlet and RSOI-induced triplet superconducting state. We also analyze the evolutions of density of states and local density of states with the weight of triplet pairing component determined by the strength of RSOI, which will be widely observed in thin films of superconductors with surface or interface-induced RSOI, or various noncentrosymmetric superconductors in terms of point contact tunneling and scanning tunneling microscopy, and thus reveal an admixture of the spin singlet and RSOI-induced triplet superconducting states.

cond-mat.supr-con↗

arXiv2010

Radiation of Terahertz Electromagnetic Waves from Build-in Nano Josephson Junctions of Cuprate High-Tc Superconductors

The nano-scale intrinsic Josephson junctions in highly anisotropic cuprate superconductors have potential for generation of terahertz electromagnetic waves. When the thickness of a superconductor sample is much smaller than the wavelength of electromagnetic waves in vacuum, the superconductor renders itself as a cavity. Unlike conventional lasers, the presence of the cavity does not guarantee a coherent emission because of the internal degree of freedom of the superconductivity phase in long junctions. We study the excitation of terahertz wave by solitons in a stack of intrinsic Josephson junctions, especially for relatively short junctions. Coherent emission requires a rectangular configuration of solitons. However such a configuration is unstable against weak fluctuations, contrarily solitons favor a triangular lattice corresponding to an out phase oscillation of electromagnetic waves. To utilize the cavity, we propose to use an array of stacks of short intrinsic Josephson junctions to generate powerful terahertz electromagnetic waves. The cavity synchronizes the plasma oscillation in different stacks and the emission intensity is predicted to be proportional to the number of stacks squared.

cond-mat.supr-con↗

arXiv2010

Phase dynamics of inductively coupled intrinsic Josephson junctions and terahertz electromagnetic radiation

The Josephson effects associated with quantum tunneling of Cooper pairs manifest as nonlinear relations between the superconductivity phase difference and the bias current and voltage. Many novel phenomena appear, such as Shapiro steps in dc cuurent-voltage (IV) characteristics of a Josephson junction under microwave shining, which can be used as a voltage standard. Inversely, the Josephson effects provide a unique way to generate high-frequency electromagnetic (EM) radiation by dc bias voltage. The discovery of cuprate high-Tc superconductors accelerated the effort to develop novel source of EM waves based on a stack of atomically dense-packed intrinsic Josephson junctions (IJJs), since the large superconductivity gap covers the whole terahertz frequency band. Very recently, strong and coherent terahertz radiations have been successfully generated from a mesa structure of $\rm{Bi_2Sr_2CaCu_2O_{8+δ}}$ single crystal which works both as the source of energy gain and as the cavity for resonance. It is then found theoretically that, due to huge inductive coupling of IJJs produced by the nanometer junction separation and the large London penetration depth of order of $\rm{μm}$ of the material, a novel dynamic state is stabilized in the coupled sine-Gordon system, in which $\pm π$ kinks in phase differences are developed responding to the standing wave of Josephson plasma and are stacked alternatively in the c-axis. This novel solution of the inductively coupled sine-Gordon equations captures the important features of experimental observations. The theory predicts an optimal radiation power larger than the one available to date by orders of magnitude, and thus suggests the technological relevance of the phenomena.

cond-mat.supr-con↗

arXiv2009

Coherent transport of armchair graphene constrictions

The coherent transport properties of armchair graphene nanoconstrictions(GNC) are studied using tight-binding approach and Green's function method. We find a non-bonding state at zero Fermi energy which results in a zero conductance valley, when a single vacancy locates at $y=3n\pm 1$ of a perfect metallic armchair graphene nanoribbon(aGNR). However, the non-bonding state doesn't exist when a vacancy locates at y=3n, and the conductance behavior of lowest conducting channel will not be affected by the vacancy. For the square-shaped armchair GNC consisting of three metallic aGNR segments, resonant tunneling behavior is observed in the single channel energy region. We find that the presence of localized edge state locating at the zigzag boundary can affect the resonant tunneling severely. A simplified one dimensional model is put forward at last, which explains the resonant tunneling behavior of armchair GNC very well.

cond-mat.mes-hall↗

arXiv2009

Effects of Pressure on the Electronic and Structural Properties of LaOFeAs

We studied the pressure effects on the electronic and structural properties of LaOFeAs by first-principles calculations. For the anti-ferromagnetic (AFM) phase with stripe- like aligned Fe spins, the electronic density of states at the Fermi level (N (EF)) slightly descends first with increasing applied pressure, then bounces up with further increasing pressure (or decreasing volume), and reaches its maximum at ~ 29.2 GPa with the volume ~ 80% of the ambient pressure value (V0). At this volume (V = 0.8V0), the LaOFeAs crystal undergoes a structural phase transition from the orthorhombic structure to the tetragonal one, which is accompanied by the disappearance of the long-ranged AFM order.

cond-mat.supr-con↗

arXiv2009

Stability of the Kink State in a Stack of Intrinsic Josephson Junctions

A new dynamic state characterized by $(2m_l+1)π$ static phase kink with integers $\{m_l\}$ is proposed recently in a stack of inductively coupled Josephson junctions. In the present paper, the stability of the phase kink state is investigated against many perturbations and it is shown that the kink state is stable. It is also discussed that the suppression of the amplitude of superconducting order parameter caused by the kink is weak.

cond-mat.supr-con↗

arXiv2009

Kink State in a Stack of Intrinsic Josephson Junctions in Layered High-$T_c$ Superconductors and Terahertz Radiation

A new family of dynamic states are found in a stack of inductively coupled intrinsic Josephson junctions in the absence of an external magnetic field. In this state, $(2m_l+1)π$ phase kinks with integers $m_l$'s stack along the c axis and lock neighboring junction together. Large dc power is pumped into plasma oscillation via kinks at the cavity resonance. The plasma oscillation is uniform along the c axis with the frequency satisfying the ac Josephson relation. Thus this state supports strong terahertz radiation and seems to be compatible with the recent experimental observations.

cond-mat.supr-con↗

arXiv2009

Comment on "Mechanism of Terahertz Electromagnetic Wave Emission from Intrinsic Josephson Junctions"

In a recent Letter, Tachiki \emph{et al.} argued that the dynamic state of superconductivity phase in multi junctions characterized by $\pmπ$ kinks is hard to realize from an energetic point of view. Instead, they proposed a so-called "state without kink". In this Comment, we point out that their energetic consideration is inconsistent with the well established treatment for Josephson junctions, and that the results derived for their "state without kink" cannot explain the radiations observed in the recent experiment.

cond-mat.supr-con↗

arXiv2009

Cavity phenomena in mesas of cuprate high-$T_c$ superconductors under voltage bias

Modeling a single crystal of cuprate high-$T_c$ superconductor, such as $\rm{Bi_2Sr_2CaCu_2O_{8+δ}}$, as a stack of intrinsic Josephson junctions, we formulate explicitly the cavity phenomenon of plasma oscillations and electromagnetic (EM) waves in mesas of cylindrical and annular shapes. When the mesa thickness is small compared with the EM wavelength, the boundary condition for the inductively coupled sine-Gordon equations is the Neumann-type one to a good approximation, addressed first theoretically and verified in a recent experiment. This renders the superconductor mesa a cavity. Biasing a dc voltage in the $c$ direction, a state with $\pmπ$ kinks in the superconductivity phase difference piled up alternatively along the c axis is stabilized. The $\pmπ$ phase kinks provide inter-lock between superconductivity phases in adjacent junctions, taking the advantage of huge inductive couplings inherent in the cuprate superconductors, which establishes the coherence across the whole system of more than $\sim 600$ junctions. They also permit a strong coupling between the lateral cavity mode and the transverse Josephson plasma, enhance the plasma oscillation significantly at the cavity modes which radiates EM waves in the terahertz band when the lateral size of mesa is set to tens of micrometers. In order to overcome the heating effect, we propose to use annular geometry. The dependence of frequency on the aspect ratio is analyzed, which reveals that the shape tailor is quite promising for improving the present technique of terahertz excitation. The annular geometry may be developed as a waveguide resonator, mimic the fiber lasers for visible lights.

cond-mat.supr-con↗

arXiv2009

Creep motions of flux lines in type II superconductors with point-like defects

We simulated the creep motions of flux lines subject to randomly distributed point-like pinning centers. It is found that at low temperatures, the pinning barrier $U$ defined in the Arrhenius-type $v-F$ characteristics increases with decreasing force $U(F) \propto F^{-μ}$, as predicted by previous theories. The exponent $μ$ is evaluated as $0.28\pm 0.02 $ for the vortex glass and $μ\simeq 0.5\pm 0.02$ for the Bragg glass (BrG). The latter is in good agreement with the prediction by the scaling theory and the functional-renormalization-group theory on creep, while the former is a new estimate. Within BrG, we find that the pinning barrier is suppressed when temperature is lifted to approximately half of the melting temperature. Characterizations of this new transition at equilibrium are also presented, indicative of a phase transition associated with the replica-symmetry breaking.

cond-mat.supr-con↗

arXiv2009

Ferromagnetism in armchair graphene nanoribbon

Due to the weak spin-orbit interaction and the peculiar relativistic dispersion in graphene, there are exciting proposals to build spin qubits in graphene nanoribbons with armchair boundaries. However, the mutual interactions between electrons are neglected in most studies so far and thus motivate us to investigate the role of electronic correlations in armchair graphene nanoribbon by both analytical and numerical methods. Here we show that the inclusion of mutual repulsions leads to drastic changes and the ground state turns ferromagnetic in a range of carrier concentrations. Our findings highlight the crucial importance of the electron-electron interaction and its subtle interplay with boundary topology in graphene nanoribbons. Furthermore, since the ferromagnetic properties sensitively depends on the carrier concentration, it can be manipulated at ease by electric gates. The resultant ferromagnetic state with metallic conductivity is not only surprising from an academic viewpoint, but also has potential applications in spintronics at nanoscale.

cond-mat.mes-hall↗

arXiv2009

Phase Dynamics in Intrinsic Josephson Junctions and its Electrodynamics

We present a theoretical description of the phase dynamics and its corresponding electrodynamics in a stack of inductively coupled intrinsic Josephson junctions of layered high-$T_c$ superconductors in the absence of an external magnetic field. Depending on the spatial structure of the gauge invariant phase difference, the dynamic state is classified into: state with kink, state without kink, and state with solitons. It is revealed that in the state with phase kink, the plasma is coupled to the cavity and the plasma oscillation is enhanced. In contrast, in the state without kink, the plasma oscillation is weak. It points a way to enhance the radiation of electromagnetic from high-$T_c$ superconductors. We also perform numerical simulations to check the theory and a good agreement is achieved. The radiation pattern of the state with and without kink is calculated, which may serve as a fingerprint of the dynamic state realized by the system. At last, the power radiation of the state with solitons is calculated by simulations. The possible state realized in the recent experiments is discussed in the viewpoint of the theoretical description. The state with kink is important for applications including terahertz generators and amplifiers.

cond-mat.supr-con↗

arXiv2008

Geometric phases and the magnetization process in quantum antiferromagnets

The physics underlying the magnetization process of quantum antiferromagnets is revisited from the viewpoint of geometric phases. A continuum variant of the Lieb-Schultz-Mattis-type approach to the problem is put forth, where the commensurability condition of Oshikawa {\it et al} derives from a Berry connection formulation of the system's crystal momentum. %, similar to that developed by Haldane for ferromagnets. %Building on the physical picture which arises, We then go on to formulate an effective field theory which can deal with higher dimensional cases as well. We find that a topological term, whose principle function is to assign Berry phase factors to space-time vortex objects, ultimately controls the magnetic behavior of the system. We further show how our effective action maps into a ${\bf Z}_2$ gauge theory under certain conditions, which in turn allows for the occurrence of a fractionalized phase with topological order.

cond-mat.stat-mech↗

arXiv2008

Three-Dimensional Phase-Kink State in Thick Stack of Josephson Junctions and Terahertz Radiation

The dynamics of superconductivity phase in thick stack of Josephson junctions with strong inductive coupling, such as the one realized in layered high-$T_c$ cuprates and possibly the recently discovered FeAs-based superconductors, is investigated under a c-axis bias voltage and in the absence of an external magnetic field. The kink state found previously by the present authors is extended to three dimensions for both rectangular and cylindrical geometries. The IV characteristics are calculated and the distributions of electromagnetic field inside the samples are clarified. The solution for a cylindrical mesa exhibits a higher resonating frequency than that of a square mesa with the same linear size by a factor of $\sim 2.4$. More importantly, from the radius dependence of the resonance frequency for the cylinder geometry it is possible to confirm directly the kink state, and thus to reveal the mechanism of the strong radiation discovered in recent experiments.

cond-mat.supr-con↗