arXiv subjects

Hao Meng

Publications and source records attributed to Hao Meng.

32 records · Page 2Linked to original sources

arXiv2020

Defect-Correlated Skyrmions and Controllable Generation in Perpendicularly Magnetized CoFeB Ultrathin Films

Skyrmions have attracted significant interest due to their topological spin structures and fascinating physical features. The skyrmion phase arises in materials with Dzyaloshinskii-Moriya (DM) interaction at interfaces or in volume of non-centrosymmetric materials. However, although skyrmions were generated experimentally, one critical intrinsic relationship between fabrication, microstructures, magnetization and the existence of skyrmions remains to be established. Here, two series of CoFeB ultrathin films with controlled atomic scale structures are employed to reveal this relationship. By inverting the growth order, the amount of defects can be artificially tuned, and skyrmions are shown to be preferentially formed at defect sites. The stable region and the density of the skyrmions can be efficiently controlled in the return magnetization loops by utilizing first-order reversal curves to reach various metastable states. These findings establish the general and intrinsic relationship from sample preparation to skyrmion generation, offering an universal method to control skyrmion density.

cond-mat.mes-hall↗

arXiv2020

Effect of Dzyaloshinskii-Moriya interaction on magnetic vortex switching driven by radial spin waves

We theoretically investigate the radial-spin-wave induced magnetic vortex switching in the presence of Dzyaloshinskii-Moriya interaction (DMI). From micromagnetic simulations, we observe a circular-to-radial vortex phase transition by increasing the DMI strength. The radial spin-wave excitation spectrum for each magnetization configuration is analyzed, showing that the frequency of spin-wave mode with a given radial node number monotonically increases (decreases) with the DMI parameter of the radial (circular) vortex. Interestingly, we find that the DMI can significantly facilitate the polarity switching of the circular vortex driven by radial spin waves. Our work provides a new insight into the DMI effect on the vortex dynamics and is helpful for designing fast all-magnonic memory devices.

cond-mat.mes-hall↗

arXiv2019

Josephson current through a ferromagnetic bilayer: Beyond the quasiclassical approximation

Based on the Bogoliubov-de Gennes equations, we provide an exact numerical solution for the critical current of Josephson junctions with a composite ferromagnetic bilayer. We demonstrate that for the antiparallel orientation of the magnetic moments of the bilayer, the presence of a potential barrier at the bilayer interface results in large oscillations of the critical current as a function of ferromagnet thickness and/or exchange field. Because of this, and remarkably, in the range of small exchange field and thicknesses, the magnetism leads to the increase of the critical current. This effect is well pronounced at low temperature but disappears near $T_c$. If the potential barrier is replaced by a spin-active barrier at the bilayer interface the conventional 0-$π$ transition, similar to the case of an uniform ferromagnetic Josephson junction, is observed. Strikingly, for a parallel orientation of the magnetic moments of the bilayer, the presence of the spin-active barrier restores the anomalous behavior---potential barrier in the antiparallel case. These behaviors result from the resonant tunneling of Cooper pairs across the composite barrier---an effect related to the spin-dependent Fermi vector in the presence of the ferromagnets' exchange field.

cond-mat.supr-con↗

arXiv2019

Magnetism induced by nonlocal spin-entangled electrons in a superconducting spin-valve

In the traditional view, the magnetic moment appearing in the superconducting region is induced by equal-spin triplet superconducting correlations in superconductor ($S$) ferromagnet ($F$) heterostructure with noncollinear magnetization. In this paper, we represent that in $NSF_1F_2$ ($N$--normal-metal) spin-valve structure the induced magnetic moment emerging in both the $S$ and $N$ regions can also be generated by Cooper pair splitting: one electron coherently tunnels from the $S$ layer into the $F_1$ layer, and the other one stays in the $S$ layer or tunnels into the $N$ layer. Two electrons are spatially separated from each other but their total spin ground state is entangled in this process. In contrast, the magnetic moment induced by the equal-spin triplet correlations hardly penetrates from the $S$ layer into the $N$ layer. In particular, by tuning the size of the exchange field and the thickness of the $F_1$ layer, one may control the direction of the induced magnetic moment in the $N$ layer. This interesting phenomenon can be attributed to the phase-shift obtained by the spin-entangled electrons. Our theoretical proposal will offer an effective way to control the entanglement of the nonlocal electrons, and also may provide possible explanations for previous and recent experimental observations [Stamopoulos et al 2005 Phys. Rev. B 72 212514; Ovsyannikov et al 2016 J. Exp. Theor. Phys. 122 738; Flokstra et al 2016 Nat. Phys. 12 57].

cond-mat.supr-con↗

arXiv2019

Nonuniform superconductivity and Josephson effect in conical ferromagnet

Using the Gorkov equations, we provide an exact solution for a one-dimensional model of superconductivity in the presence of a conical helicoidal exchange field. Due to the special type of symmetry of the system, the superconducting transition always occurs into a nonuniform superconducting phase (in contrast with the Fulde-Ferrell-Larkin-Ovchinnikov state, which appears only at low temperatures). We directly demonstrate that the uniform superconducting state in our model carries a current and thus does not correspond to the ground state. We study in the framework of the Bogoliubov-de Gennes approach the properties of the Josephson junction with a conical ferromagnet as a weak link. In our numerical calculations, we do not use any approximations (such as, e.g., a quasiclassical approach), and we show a realization of an anomalous $ϕ_{0}$ junction (with a spontaneous phase difference $ϕ_{0}$ in the ground state). The spontaneous phase difference $ϕ_{0}$ strongly increases at high values of the exchange field near the borderline with a half-metal, and it exists also in the half-metal regime.

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arXiv2018

Symmetric thermal optimal path and time-dependent lead-lag relationship: Novel statistical tests and application to UK and US real-estate and monetary policies

We present the symmetric thermal optimal path (TOPS) method to determine the time-dependent lead-lag relationship between two stochastic time series. This novel version of the previously introduced TOP method alleviates some inconsistencies by imposing that the lead-lag relationship should be invariant with respect to a time reversal of the time series after a change of sign. This means that, if `$X$ comes before $Y$', this transforms into `$Y$ comes before $X$' under a time reversal. We show that previously proposed bootstrap test lacks power and leads too often to a lack of rejection of the null that there is no lead-lag correlation when it is present. We introduce instead two novel tests. The first the free energy p-value $ρ$ criterion quantifies the probability that a given lead-lag structure could be obtained from random time series with similar characteristics except for the lead-lag information. The second self-consistent test embodies the idea that, for the lead-lag path to be significant, synchronizing the two time series using the time varying lead-lag path should lead to a statistically significant correlation. We perform intensive synthetic tests to demonstrate their performance and limitations. Finally, we apply the TOPS method with the two new tests to the time dependent lead-lag structures of house price and monetary policy of the United Kingdom (UK) and United States (US) from 1991 to 2011. The TOPS approach stresses the importance of accounting for change of regimes, so that similar pieces of information or policies may have drastically different impacts and developments, conditional on the economic, financial and geopolitical conditions. This study reinforces the view that the hypothesis of statistical stationarity is highly questionable.

q-fin.ST↗

arXiv2016

Long-range supercurrents induced by the interference effect of opposite-spin triplet state in clean superconductor-ferromagnet structures

By now it is known that in an s-wave superconductor-ferromagnet-superconductor ($SFS$) structure the supercurrent induced by spin singlet pairs can only transmit a short distance of the order of magnetic coherence length. The long-range supercurrent, taking place on the length scale of the normal metal coherence length, will be maintained by equal-spin triplet pairs, which can be generated by magnetic inhomogeneities in the system. In this paper, we show an unusual long-range supercurrent, which can take place in clean $SF_1F_2S$ junction with non-parallel orientation of magnetic moments. The mechanism behind the enhancement of Josephson current is provided by the interference of the opposite-spin triplet states derived from $S/F_1$ and $F_2/S$ interfaces when both ferromagnetic layers have the same values of the length and exchange field. This finds can provide a natural explanation for recent experiment [Robinson et al., Phys. Rev. Lett. 104, 207001 (2010)].

cond-mat.supr-con↗

arXiv2015

Long-range superharmonic Josephson current and spin-triplet pairing correlations in a junction with ferromagnetic bilayers

The long-range spin-triplet supercurrent transport is an interesting phenomenon in the superconductor/ferromagnet ($S/F$) heterostructure containing noncollinear magnetic domains. Here we study the long-range superharmonic Josephson current in asymmetric $S/F_1/F_2/S$ junctions. It is demonstrated that this current is induced by spin-triplet pairs $\mid\uparrow\uparrow\rangle-$$\mid\downarrow\downarrow\rangle$ or $\mid\uparrow\uparrow\rangle+$$\mid\downarrow\downarrow\rangle$ in the thick $F_1$ layer. The magnetic rotation of the particularly thin $F_2$ layer will not only modulate the amplitude of the superharmonic current but also realise the conversion between $\mid\uparrow\uparrow\rangle-$$\mid\downarrow\downarrow\rangle$ and $\mid\uparrow\uparrow\rangle+$$\mid\downarrow\downarrow\rangle$. Moreover, the critical current shows an oscillatory dependence on thickness and exchange field in the $F_2$ layer. These effect can be used for engineering cryoelectronic devices manipulating the superharmonic current. In contrast, the critical current declines monotonically with increasing exchange field of the $F_1$ layer, and if the $F_1$ layer is converted into half-metal, the long-range supercurrent is prohibited but $\mid\uparrow\uparrow\rangle$ still exists within the entire $F_1$ region. This phenomenon contradicts the conventional wisdom and indicates the occurrence of spin and charge separation in present junction, which could lead to useful spintronics devices.

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arXiv2015

Club Convergence of House Prices: Evidence from China's Ten Key Cities

The latest global financial tsunami and its follow-up global economic recession has uncovered the crucial impact of housing markets on financial and economic systems. The Chinese stock market experienced a markedly fall during the global financial tsunami and China's economy has also slowed down by about 2\%-3\% when measured in GDP. Nevertheless, the housing markets in diverse Chinese cities seemed to continue the almost nonstop mania for more than ten years. However, the structure and dynamics of the Chinese housing market are less studied. Here we perform an extensive study of the Chinese housing market by analyzing ten representative key cities based on both linear and nonlinear econophysical and econometric methods. We identify a common collective driving force which accounts for 96.5\% of the house price growth, indicating very high systemic risk in the Chinese housing market. The ten key cities can be categorized into clubs and the house prices of the cities in the same club exhibit an evident convergence. These findings from different methods are basically consistent with each other. The identified city clubs are also consistent with the conventional classification of city tiers. The house prices of the first-tier cities grow the fastest, and those of the third- and fourth-tier cities rise the slowest, which illustrates the possible presence of a ripple effect in the diffusion of house prices in different cities.

q-fin.ST↗

arXiv2014

Peculiar long-range supercurrent in SFS junction containing a noncollinear magnetic domain in the ferromagnetic region

We study the supercurrent in a superconductor-ferromagnet-superconductor heterostructure containing a noncollinear magnetic domain in the ferromagnetic region. It is demonstrated that the magnetic domain can lead to a spin-flip process, which can reverse the spin orientations of the singlet Cooper pair propagating through the magnetic domain region. If the ferromagnetic layers on both sides of magnetic domain have the same features, the long-range proximity effect will take place. That is because the singlet Cooper pair will create an exact phase-cancellation effect and gets an additional $π$ phase shift as it passes through the entire ferromagnetic region. Then the equal spin triplet pair only exists in the magnetic domain region and can not diffuse into the other two ferromagnetic layers. So the supercurrent mostly arises from the singlet Cooper pairs and the equal spin triplet pairs are not involved. This behavior is quite distinct from the common knowledge that long-range supercurrent induced by inhomogeneous ferromagnetism stems from the equal spin triplet pairs. The result we presented here provides a new way for generating the long-range supercurrent.

cond-mat.supr-con↗

arXiv2013

Long-Range triplet Josephson Current driven by the bias voltage

We study the long-range triplet Josephson current in a clean junction composed of two s-wave superconductors and a normal-metal/ferromagnet/normal-metal trilayer. Through applying the bias voltages on the metal regions by two antiparallel half-metal electrodes, we show that the amplitude and direction of this long-range current can be controlled easily and flexibly. Such current arises from the fact that the applied voltage can produce a nonequilibrium spin-dependent quasiparticle distribution in the metal regions so that the Cooper pairs acquire an extra momenta, which will lead to a spin-flip processes in the metal regions. This processes can produce the parallel spin triplet pairs in the central ferromagnet layer. In particular, if the voltage is applied only on one metal region, we further find that the recently discovered long-range superharmonic Josephson current will appear because of the transport of an even number of parallel spin triplet pairs.

cond-mat.supr-con↗

arXiv2013

Long-Range triplet Josephson Current Modulated by the Interface Magnetization Texture

We have investigated the Josephson coupling between two s-wave superconductors separated by the ferromagnetic trilayers with noncollinear magnetization. We find that the long-range triplet critical current will oscillate with the strength of the exchange field and the thickness of the interface layer, when the interface magnetizations are orthogonal to the central magnetization. This feature is induced by the spatial oscillations of the spin-triplet state |\uparrow\downarrow>+|\downarrow\uparrow> in the interface layer. Moreover, the critical current can exhibit a characteristic nonmonotonic behavior, when the misalignment angle between interface magnetization and central ferromagnet increases from 0 to π/2. This peculiar behavior will take place under the condition that the original state of the junction with the parallel magnetizations is the πstate.

cond-mat.supr-con↗

arXiv2013

Systemic risk and spatiotemporal dynamics of the US housing market

Housing markets play a crucial role in economies and the collapse of a real-estate bubble usually destabilizes the financial system and causes economic recessions. We investigate the systemic risk and spatiotemporal dynamics of the US housing market (1975-2011) at the state level based on the Random Matrix Theory (RMT). We identify rich economic information in the largest eigenvalues deviating from RMT predictions and unveil that the component signs of the eigenvectors contain either geographical information or the extent of differences in house price growth rates or both. Our results show that the US housing market experienced six different regimes, which is consistent with the evolution of state clusters identified by the box clustering algorithm and the consensus clustering algorithm on the partial correlation matrices. Our analysis uncovers that dramatic increases in the systemic risk are usually accompanied with regime shifts, which provides a means of early detection of housing bubbles.

q-fin.ST↗

arXiv2012

Effects of long memory in the order submission process on the properties of recurrence intervals of large price fluctuations

Understanding the statistical properties of recurrence intervals of extreme events is crucial to risk assessment and management of complex systems. The probability distributions and correlations of recurrence intervals for many systems have been extensively investigated. However, the impacts of microscopic rules of a complex system on the macroscopic properties of its recurrence intervals are less studied. In this Letter, we adopt an order-driven stock market model to address this issue for stock returns. We find that the distributions of the scaled recurrence intervals of simulated returns have a power law scaling with stretched exponential cutoff and the intervals possess multifractal nature, which are consistent with empirical results. We further investigate the effects of long memory in the directions (or signs) and relative prices of the order flow on the characteristic quantities of these properties. It is found that the long memory in the order directions (Hurst index $H_s$) has a negligible effect on the interval distributions and the multifractal nature. In contrast, the power-law exponent of the interval distribution increases linearly with respect to the Hurst index $H_x$ of the relative prices, and the singularity width of the multifractal nature fluctuates around a constant value when $H_x<0.7$ and then increases with $H_x$. No evident effects of $H_s$ and $H_x$ are found on the long memory of the recurrence intervals. Our results indicate that the nontrivial properties of the recurrence intervals of returns are mainly caused by traders' behaviors of persistently placing new orders around the best bid and ask prices.

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