Ke Xia

Ke Xia
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Ke Xia

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Physics - Fluid Dynamics (19)
Physics - Materials Science (18)
Physics - Mesoscopic Systems and Quantum Hall Effect (18)
Nonlinear Sciences - Chaotic Dynamics (1)
Physics - Geophysics (1)
Physics - Other (1)

Publications Authored By Ke Xia

We present a systematic investigation of the effects of roughness geometry on turbulent Rayleigh-B\'enard convection (RBC) over rough plates with pyramid-shaped and periodically distributed roughness elements. Using a parameter $\lambda$ defined as the height of a roughness element over its base width, the heat transport, the flow dynamics and local temperatures are measured for the Rayleigh number range $7.50\times 10^{7} \leq Ra\leq 1. Read More

We study magnon-photon coupling in cavity in the presence of relative phase shift between magnetic and electric components of the microwave. We show that the anticrossing gap can be manipulated by varying the relative phase. Increasing the phase difference leads to narrowing the anticrossing gap of hybridized modes and eventually to phase locked coupling at the value of relative phase close to $\pi$. Read More

Many natural and engineering systems are simultaneously subjected to a driving force and a stabilizing force. The interplay between the two forces, especially for highly nonlinear systems such as fluid flow, often results in surprising features. Here we reveal such features in three different types of Rayleigh-B\'enard (RB) convection, i. Read More

The effects of insulating lids on the convection beneath were investigated experimentally using rectangular convection cells in the flux Rayleigh number range $2.3\times10^{9}\leq Ra_F \leq 1.8\times10^{11}$ and cylindrical cells in the range $1. Read More

In this paper we investigate a hybrid quantum system comprising a mechanical oscillator coupled via magnetic induced electromotive force to an $LC$ resonator. We derive the Lagrangian and Hamiltonian for this system and find that the interaction can be described by a charge-momentum coupling with a strength that has a strong geometric dependence. We focus our study on a mechanical resonator with a thin-film magnetic coating which interacts with a nano-fabricated planar coil. Read More

Yttrium Iron Garnet is the ubiquitous magnetic insulator used for studying pure spin currents. The exchange constants reported in the literature vary considerably between different experiments and fitting procedures. Here we calculate them from first-principles. Read More

We propose a new scaling law for anomalous Hall effect in ferromagnetic thin films by distinguishing three scattering sources, namely, bulk impurity, phonon, and more importantly a rough surface. This new scaling law fits the recent experimental data excellently with constant coefficients that are independent of temperature and film thickness. This is in stark constrast with previous scaling laws that use temperature/thickness dependent fitting coefficients, and is a strong indicator that this law captures the essential physics. Read More

We derive the complete expression of nonlocal damping in noncollinear magnetization due to the nonuniform spin current pumped by precessional magnetization and incorporate it into a generalized Thiele equation to study its effects on the dynamics of the transverse and vortex domain walls (DWs) in ferromagnetic nanowires. We demonstrate that the transverse component of nonlocal damping slows down the field-driven DW propagation and increases the Walker breakdown field whereas it is neglected in many previous works in literature. The experimentally measured DW mobility variation with the damping tuned by doping with heavy rare-earth elements that had discrepancy from micromagnetic simulation are now well understood with the nonlocal damping. Read More

The spin Hall angle (SHA) is a measure of the efficiency with which a transverse spin current is generated from a charge current by the spin-orbit coupling and disorder in the spin Hall effect (SHE). In a study of the SHE for a Pt$|$Py (Py=Ni$_{80}$Fe$_{20}$) bilayer using a first-principles scattering approach, we find a SHA that increases monotonically with temperature and is proportional to the resistivity for bulk Pt. By decomposing the room temperature SHE and inverse SHE currents into bulk and interface terms, we discover a giant interface SHA that dominates the total inverse SHE current with potentially major consequences for applications. Read More

We present an experimental investigation of the dynamics and flow-coupling of convective turbulent flows in a cylindrical Rayleigh-Benard convection cell with two immiscible fluids, water and fluorinert FC-77 electronic liquid (FC77). It is found that one large-scale circulation (LSC) roll exists in each of the fluid layers, and that their circulation planes have two preferred azimuthal orientations separated by $\sim\pi$. A surprising finding of the study is that cessations/reversals of the LSC in FC77 of the two-layer system occur much more frequently than they do in single-layer turbulent RBC, and that a cessation is most likely to result in a flow reversal of the LSC, which is in sharp contrast with the uniform distribution of the orientational angular change of the LSC before and after cessations in single-layer turbulent RBC. Read More

We present experimental evidence that a minute amount of polymer additives can significantly enhance heat transport in the bulk region of turbulent thermal convection. The effects of polymer additives are found to be the \textit{suppression} of turbulent background fluctuations that give rise to incoherent heat fluxes that make no net contribution to heat transport, and at the same time to \textit{increase} the coherency of temperature and velocity fields. The suppression of small-scale turbulent fluctuations leads to more coherent thermal plumes that result in the heat transport enhancement. Read More

We studied the thermoelectric coefficients (Seebeck and thermal conductance)of FeCo|MgO|FeCo(001) magnetic tunnel junctions (MTJs) from first principles using a generalized Landauer-B\"{u}ttiker formalism. FeCo|MgO|FeCo(001) MTJs usually yield smaller thermoelectric effects compared with epitaxial Fe|MgO|Fe(001) MTJs. The (magneto-) Seebeck effect is sensitive to the details of the FeCo$|$MgO interfaces. Read More

We report an experimental study of the properties of the velocity boundary layer in turbulent Rayleigh-B\'{e}nard convection in a cylindrical cell. The measurements were made at Rayleigh numbers $Ra$ in the range $2.8\times10^{8}Read More

We examine the behavior of the Kolmogorov constants C_2, C_k, and C_{k1}, which are, respectively, the prefactors of the second order longitudinal structure function, the three dimensional and one-dimensional longitudinal energy spectrum in the inertial range. We show that their ratios, C_2/C_{k1} and C_k/C_{k1}, exhibit clear dependence on the micro-scale Reynolds number R_{\lambda}, implying that they cannot all be independent of R_{\lambda}. In particular, it is found that (C_{k1}/C_2-0. Read More

We report an experimental investigation of pair dispersions in turbulent thermal convection with initial separation r0 ranging from sub-Kolmogorov scale to scales in the inertial range. In the dissipative range of scales we observed for the first time in experiment the exponential growth of the separation between a pair of particles predicted by Batchelor and obtained a Batchelor constant 0.23\pm0. Read More

We compute the shot noise in ballistic and disordered Fe/MgO/Fe tunnel junctions by a wave function-matching method. For tunnel barriers with no more than 5 atomic layers we find a suppression of the Fano factor as a function of the magnetic configuration. For thicker MgO barriers the shot noise is suppressed up to a threshold bias indicating the onset of resonant tunneling. Read More

We numerically investigate the structures of the near-plate temperature profiles close to the bottom and top plates of turbulent Rayleigh-B\'{e}nard flow in a cylindrical sample at Rayleigh numbers Ra=10^8 to Ra=2\times10^{12} and Prandtl numbers Pr=6.4 and Pr=0.7 with the dynamical frame method [Q. Read More

We analyze the reversals of the large scale flow in Rayleigh-B\'enard convection both through particle image velocimetry flow visualization and direct numerical simulations (DNS) of the underlying Boussinesq equations in a (quasi) two-dimensional, rectangular geometry of aspect ratio 1. For medium Prandtl number there is a diagonal large scale convection roll and two smaller secondary rolls in the two remaining corners diagonally opposing each other. These corner flow rolls play a crucial role for the large scale wind reversal: They grow in kinetic energy and thus also in size thanks to plume detachments from the boundary layers up to the time that they take over the main, large scale diagonal flow, thus leading to reversal. Read More

We investigate the structures of the near-plate velocity and temperature profiles at different horizontal positions along the conducting bottom (and top) plate of a Rayleigh-B\'{e}nard convection cell, using two-dimensional (2D) numerical data obtained at the Rayleigh number Ra=10^8 and the Prandtl number Pr=4.4 of an Oberbeck-Boussinesq flow with constant material parameters. The results show that most of the time, and for both velocity and temperature, the instantaneous profiles scaled by the dynamical frame method [Q. Read More

Spin valves incorporating perpendicularly magnetized materials are promising structures for memory elements and high-frequency generators. We report the angular dependence of the spin-transfer torque in spin valves with perpendicular equilibrium magnetization computed by first-principles circuit theory and compare results with experiments by W.H. Read More

The influence of the surface structure and vibration mode on the resistivity of Cu films and the corresponding size effect are investigated. The temperature dependent conductivities of the films with different surface morphologies are calculated by the algorithm based upon the tight-binding linear muffin-tin orbital method and the Green's function technique. The thermal effect is introduced by setting the atomic displacements according to the Gaussian distribution with the mean-square amplitude estimated by the Debye model. Read More

We compute thermal spin transfer torques (TST) in Fe-MgO-Fe tunnel junctions using a first principles wave function-matching method. At room temperature, the TST in a junction with 3 MgO monolayers amounts to 10^-7J/m^2/K, which is estimated to cause magnetization reversal for temperature differences over the barrier of the order of 10 K. The large TST can be explained by multiple scattering between interface states through ultrathin barriers. Read More

We report first-principles analysis on the bias dependence of spin-transfer torque (STT) in Fe/MgO/Fe magnetic tunnel junctions. The in-plane STT changes from linear to nonlinear dependence as the bias voltage is increased from zero. The angle dependence of STT is symmetric at low bias but asymmetric at high bias. Read More

Recent experimental and theoretical studies focus on spin-mediated heat currents at interfaces between normal metals and magnetic insulators. We resolve conflicting estimates for the order of magnitude of the spin transfer torque by first-principles calculations. The spin mixing conductance G^\uparrow\downarrow of the interface between silver and the insulating ferrimagnet Yttrium Iron Garnet (YIG) is dominated by its real part and of the order of 10^14 \Omega^-1m^-2, i. Read More

We present a method of disentangling the anisotropies produced by the cliff structures in turbulent velocity field and test it in the system of turbulent Rayleigh-B\'{e}nard (RB) convection. It is found that in the RB system the cliff structures in the velocity field are generated by thermal plumes. These cliff structures induce asymmetry in the velocity increments, which leads us to consider the plus and minus velocity structure functions (VSF). Read More

Current-perpendicular-to-plane magnetoresistance (CPP-MR) measurements let us quantify conduction electron scattering and spin-flipping at a sputtered ferromagnetic/ferromagnetic (F1/F2 = Co/Ni) interface, with important consequences for CPP-MR and spin-torque experiments with perpendicular anisotropy. We use ferromagnetically coupled ([Ni/Co]xn)Ni multilayers, and Py-based, symmetric double exchange-biased spin-valves (DEBSVs) containing inserts of ferromagnetically coupled ([Co/Ni]xn)Co or ([Ni/Co]xn)Ni multilayers, to derive Co/Ni interface specific resistances AR(Co/Ni)(Up) = 0.03 (+0. Read More

We present a systematic experimental study of geometric and statistical properties of thermal plumes in turbulent Rayleigh-B\'{e}nard convection using the thermochromic-liquid-crystal (TLC) technique. The experiments were performed in three water-filled cylindrical convection cells with aspect ratios 2, 1, and 0.5 and over the Rayleigh-number range $5\times10^7 \leq Ra \leq 10^{11}$. Read More

The shape of velocity and temperature profiles near the horizontal conducting plates in turbulent Rayleigh-B\'{e}nard convection are studied numerically and experimentally over the Rayleigh number range $10^8\lesssim Ra\lesssim3\times10^{11}$ and the Prandtl number range $0.7\lesssim Pr\lesssim5.4$. Read More

We report measurements of the instantaneous viscous boundary layer (BL) thickness $\delta_v(t)$ in turbulent Rayleigh-B\'{e}nard convection. It is found that $\delta_v(t)$ obtained from the measured instantaneous two-dimensional velocity field exhibits intermittent fluctuations. For small values, $\delta_v(t)$ obeys a lognormal distribution, whereas for large values the distribution of $\delta_v(t)$ exhibits an exponential tail. Read More

We have measured an Ag/Al interface specific resistance, 2AR(Ag/Al)(111) = 1.4 fOhm-m^2, that is twice that predicted for a perfect interface, 50% larger than for a 2 ML 50%-50% alloy, and even larger than our newly predicted 1.3 fOhmm^2 for a 4 ML 50%-50% alloy. Read More

From measurements of the current-perpendicular-to-plane (CPP) total specific resistance (AR = area times resistance) of sputtered Pd/Ir multilayers, we derive the interface specific resistance, 2AR(Pd/Ir) = 1.02 +/- 0.06 fOhmm^2, for this metal pair with closely similar lattice parameters. Read More

We report an experimental study of the large-scale circulation (LSC) in a turbulent Rayleigh-B\'{e}nard convection cell with aspect ratio unity. The temperature-extremum-extraction (TEE) method for obtaining the dynamic information of the LSC is presented. With this method, the azimuthal angular positions of the hot ascending and cold descending flows along the sidewall are identified from the measured instantaneous azimuthal temperature profile. Read More

We report an experimental study of the three-dimensional spatial structure of the low frequency temperature oscillations in a cylindrical Rayleigh-B\'{e}nard convection cell. It is found that thermal plumes are not emitted periodically, but randomly and continuously, from the top and bottom plates. We further found that the oscillation of the temperature field does not originate from the boundary layers, but rather is a result of the horizontal motion of the hot ascending and cold descending fluids being modulated by the twisting and sloshing motion of the bulk flow field. Read More

We investigate experimentally the statistical properties of active and passive scalar fields in turbulent Rayleigh-B\'{e}nard convection in water, at $Ra\sim10^{10}$. Both the local concentration of fluorescence dye and the local temperature are measured near the sidewall of a rectangular cell. It is found that, although they are advected by the same turbulent flow, the two scalars distribute differently. Read More

We report a theoretical study on the spin and electron transport in the nonlocal lateral spin valve with non-collinear magnetic configuration. The nonlocal magnetoresistance, defined as the voltage difference on the detection lead over the injected current, is derived analytically. The spin transfer torques on the detection lead are calculated. Read More

An efficient first principles method was developed to calculate spin transfer torques in layered system with noncollinear magnetization. The complete scattering wave function is determined by matching the wave function in the scattering region with the Bloch states in the leads. The spin transfer torques are obtained with aid of the scattering wave function. Read More

We report a first principles formalism and its numerical implementation for treating quantum transport properties of nanoelectronic devices with atomistic disorder. We developed a nonequilibrium vertex correction (NVC) theory to handle the configurational average of random disorder at the density matrix level so that disorder effects to nonlinear and nonequilibrium quantum transport can be calculated from atomic first principles in a self-consistent and efficient manner. We implemented the NVC into a Keldysh non-equilibrium Green's function (NEGF) based density functional theory (DFT) and applied the NEGF-DFT-NVC formalism to magnetic tunnel junctions with interface roughness disorder. Read More

The spin dependent reflection at the interface is the key element to understand the spin transport. By completely solving the scattering problem based on first principles method, we obtained the spin resolved reflectivity spectra. The comparison of our theoretical results with experiment is good in a large energy scale from Fermi level to energy above vacuum level. Read More

In spite of the absence of a macroscopic magnetic moment, an anti-ferromagnet is spin-polarized on an atomic scale. The electric current passing through a conducting anti-ferromagnet is polarized as well, leading to spin-transfer torques when the order parameter is textured, such as in anti-ferromagnetic non-collinear spin valves and domain walls. We report a first principles study on the electronic transport properties of anti-ferromagnetic systems. Read More

The time reversal Aharonov-Casher (AC) interference effect in the mesoscopic ring structures, based on the experiment in Phys. Rev. Lett. Read More

Temperature and velocity oscillations have been found in a rectangular Rayleigh-B\'{e}nard convection cell, in which one large scale convection roll exists. At $Ra=8.9 \times 10^{11}$ and $Pr=4$, temperature oscillation can be observed in most part of the system and the oscillation period remains almost constant, $t_T = 74 \pm 2$ seconds. Read More

By carrying out density functional theory analysis within the Keldysh non-equilibrium Green's functional formalism, we have calculated the nonlinear and non-equilibrium quantum transport properties of Fe/MgO/Fe trilayer structures as a function of external bias voltage. For well relaxed atomic structures of the trilayer, the equilibrium tunnel magnetoresistance ratio (TMR) is found to be very large and also fairly stable against small variations in the atomic structure. As a function of external bias voltage, the TMR reduces monotonically to zero with a voltage scale of about 1V, in agreement with experimental observations. Read More

Stimulated generally by recent interest in the novel spin Hall effect, the nonrelativistic quantum mechanical conserved currents, taken into account of spin-orbit coupling, are rigorously formulated based on the symmetries of system and Noether' theorem. The quantum mechanical force on the spin as well as the torque associated with the variance of angular momentum are obtained. Consequently, the kinetic interpretation of the variances of spin and orbit angular momentum currents implies a torque on the "electric dipole" associated with the moving spin. Read More

The theory of charge and spin transport in forward-biased Schottky barriers reveals characteristic and experimentally relevant features. The conductance mismatch is found to enhance the current induced spin-imbalance in the semiconductor. The GaAs|MnAs interface resistance is obtained from an analysis of the magnetic field dependent Kerr rotation experiments by Stephens et al. Read More

We present a scheme to extract information about plumes, a prominent coherent structure in turbulent thermal convection, from simultaneous local velocity and temperature measurements. Using this scheme, we study the temperature dependence of the plume velocity and understand the results using the equations of motion. We further obtain the average local heat flux in the vertical direction at the cell center. Read More

The scattering theory of transport has to be applied with care in a diffuse environment. Here we discuss how the scattering matrices of heterointerfaces can be used to compute interface resistances of dirty magnetic multilayers. First principles calculations of these interface resistances agree well with experiments in the CPP (current perpendicular to the interface plane) configuration. Read More

Semiclassical theories of electron and spin transport in metallic magnetic structures are reviewed with emphasis on the role of disorder and electronic band structures in the current perpendicular to the interface plane (CPP) transport configuration. Read More