Asymptotic Analysis for Low-Resolution Massive MIMO Systems with MMSE Receiver

The uplink achievable rate of massive multiple- input-multiple-output (MIMO) systems, where the low-resolution analog-to-digital converters (ADCs) are assumed to equip at the base station (BS), is investigated in this paper. We assume that only imperfect channel station information is known at the BS. Then a new MMSE receiver is designed by taking not only the Gaussian noise, but also the channel estimation error and quantizer noise into account. By using the Stieltjes transform of random matrix, we further derive a tight asymptotic equivalent for the uplink achievable rate with proposed MMSE receiver. We present a detailed analysis for the number of BS antennas through the expression of the achievable rates and validate the results using numerical simulations. It is also shown that we can compensate the performance loss due to the low-resolution quantization by increasing the number of antennas at the BS.Comment: 7 pages, 3 figure

First-principles study the electronic and structural properties of chromium arsenide

We systematically study chromium arsenide in various crystal structures in order to investigate the structural, magnetic and electronic properties for real applications. The calculations are performed within the density functional framework using the projected augmented plane wave method as employed in VASP code. Using the generalized gradient approximation (GGA) for exchange correlation energy functional, we have calculated the lattice parameters, bulk modulus and cohesive energy. Here we show the structural parameters and magnetic properties of CrAs in six different structures. The density of states are calculated and differences are compared. Our results for structural and electronic properties are compared with the experimental and other theoretical results wherever these are availableComment: 14, 6 figure

Magnetic interactions in a proposed diluted magnetic semiconductor (Ba1-x_\text{1-x}Kx_\text{x})(Zn1-y_\text{1-y}Mny_\text{y})2_\text{2}P2_\text{2}

By using first-principles electronic structure calculations, we have studied the magnetic interactions in a proposed BaZn$_2$P$_2$-based diluted magnetic semiconductor (DMS). For a typical compound Ba(Zn$_{0.944}$Mn$_{0.056}$)$_2$P$_2$ with only spin doping, due to the superexchange interaction between Mn atoms and the lack of itinerant carriers, the short-range antiferromagnetic coupling dominates. Partially substituting K atoms for Ba atoms, which introduces itinerant hole carriers into the $p$ orbitals of P atoms so as to link distant Mn moments with the spin-polarized hole carriers via the $p$-$d$ hybridization between P and Mn atoms, is very crucial for the appearance of ferromagnetism in the compound. Furthermore, applying hydrostatic pressure first enhances and then decreases the ferromagnetic coupling in (Ba$_{0.75}$K$_{0.25}$)(Zn$_{0.944}$Mn$_{0.056}$)$_2$P$_2$ at a turning point around 15 GPa, which results from the combined effects of the pressure-induced variations of electron delocalization and $p$-$d$ hybridization. Compared with the BaZn$_2$As$_2$-based DMS, the substitution of P for As can modulate the magnetic coupling effectively. Both the results for BaZn$_2$P$_2$-based and BaZn$_2$As$_2$-based DMSs demonstrate that the robust antiferromagnetic (AFM) coupling between the nearest Mn-Mn pairs bridged by anions is harmful to improving the performance of this II-II-V based DMS materials.Comment: 7 pages, 6 figures, 1 table; Accepted by Chinese Physics B (2018

On the origin of the Extreme-Ultraviolet late phase of solar flares

Solar flares typically have an impulsive phase that followed by a gradual phase as best seen in soft X-ray emissions. A recent discovery based on the EUV Variability Experiment (EVE) observations onboard the Solar Dynamics Observatory (SDO) reveals that some flares exhibit a second large peak separated from the first main phase peak by tens of minutes to hours, which is coined as the flare's EUV late phase. In this paper, we address the origin of the EUV late phase by analyzing in detail two late phase flares, an M2.9 flare on 2010 October 16 and an M1.4 flare on 2011 February 18, using multi-passband imaging observations from the Atmospheric Imaing Assembly (AIA) onboard SDO. We find that: (1) the late phase emission originates from a different magnetic loop system, which is much larger and higher than the main phase loop system. (2) The two loop systems have different thermal evolution. While the late phase loop arcade reaches its peak brightness progressively at a later time spanning for more than one hour from high to low temperatures, the main phase loop arcade reaches its peak brightness at almost the same time (within several minutes) in all temperatures. (3) Nevertheless, the two loop systems seem to be connected magnetically, forming an asymmetric magnetic quadruple configuration. (4) Further, the footpoint brightenings in UV wavelengths show a systematic delay of about one minute from the main flare region to the remote footpoint of the late phase arcade system. We argue that the EUV late phase is the result of a long-lasting cooling process in the larger magnetic arcade system.Comment: 12 figure

Extremely Large EUV Late Phase of Solar Flares

The second peak in the Fe XVI 33.5 nm line irradiance observed during solar flares by Extreme ultraviolet Variability Experiment (EVE) is known as Extreme UltraViolet (EUV) late phase. Our previous paper (Liu et al. 2013) found that the main emissions in the late phase are originated from large-scale loop arcades that are closely connected to but different from the post flare loops (PFLs), and we also proposed that a long cooling process without additional heating could explain the late phase. In this paper, we define the extremely large late phase because it not only has a bigger peak in the warm 33.5 irradiance profile, but also releases more EUV radiative energy than the main phase. Through detailedly inspecting the EUV images from three point-of-view, it is found that, besides the later phase loop arcades, the more contribution of the extremely large late phase is from a hot structure that fails to erupt. This hot structure is identified as a flux rope, which is quickly energized by the flare reconnection and later on continuously produces the thermal energy during the gradual phase. Together with the late-phase loop arcades, the fail to erupt flux rope with the additional heating create the extremely large EUV late phase.Comment: 9 figure

Perfect charge compensation in extremely large magnetoresistance materials LaSb and LaBi revealed by the first-principles calculations

By the first-principles electronic structure calculations, we have systematically studied the electronic structures of recently discovered extremely large magnetoresistance (XMR) materials LaSb and LaBi. We find that both LaSb and LaBi are semimetals with the electron and hole carriers in perfect balance. The calculated carrier densities in the order of $10^{20}$ cm$^{-3}$ are in good agreement with the experimental values, implying long mean free time of carriers and thus high carrier mobilities. With a semiclassical two-band model, the perfect charge compensation and high carrier mobilities naturally explain (i) the XMR observed in LaSb and LaBi; (ii) the non-saturating quadratic dependence of XMR on external magnetic field; and (iii) the resistivity plateau in the turn-on temperature behavior at very low temperatures. The explanation of these features without resorting to the topological effect indicates that they should be the common characteristics of all perfectly electron-hole compensated semimetals.Comment: 7 pages, 7 figures, 1 tabl

Pressure-induced topological phase transition in LaSb: First-principles study

By using first-principles electronic structure calculations, we predict that the extreme magnetoresistance (XMR) material LaSb takes a topological phase transition without breaking any symmetry under a hydrostatic pressure applied between 3 and 4 GPa, meanwhile the electron-hole compensation remains in its electronic band structure. Thus LaSb provides an ideal platform for studying the individual role of topological property playing in the XMR phenomenon, in addition to the electron-hole compensation. This has general implication to the relationship of XMR effect and topological property in topological materials.Comment: 6 pages, 4 figures, 2 table

The melilite-type compound (Sr1−x_{1-x},AxA_x)2_2MnGe2_2S6_6O (AA=K, La) being a room temperature ferromagnetic semiconductor

The seeking of room temperature ferromagnetic semiconductors, which take advantages of both the charge and spin degrees of freedom of electrons to realize a variety of functionalities in devices integrated with electronic, optical, and magnetic storage properties, has been a long-term goal of scientists and engineers. Here, by using the spin-polarized density functional theory calculations, we predict a new series of high temperature ferromagnetic semiconductors based on the melilite-type oxysulfide Sr$_2$MnGe$_2$S$_6$O through hole (K) and electron (La) doping. Due to the lack of strong antiferromagnetic superexchange between Mn ions, the weak antiferromagnetic order in the parent compound Sr$_2$MnGe$_2$S$_6$O can be suppressed easily by charge doping with either $p$-type or $n$-type carriers, giving rise to the expected ferromagnetic order. At a doping concentration of 25%, both the hole-doped and electron-doped compounds can achieve a Curie temperature ($T_\text{c}$) above 300 K. The underlying mechanism is analyzed. Our study provides an effective approach for exploring new types of high temperature ferromagnetic semiconductors.Comment: 6 pages, 4 figures, 1 tabl

Guaranteed Sufficient Decrease for Variance Reduced Stochastic Gradient Descent

In this paper, we propose a novel sufficient decrease technique for variance reduced stochastic gradient descent methods such as SAG, SVRG and SAGA. In order to make sufficient decrease for stochastic optimization, we design a new sufficient decrease criterion, which yields sufficient decrease versions of variance reduction algorithms such as SVRG-SD and SAGA-SD as a byproduct. We introduce a coefficient to scale current iterate and satisfy the sufficient decrease property, which takes the decisions to shrink, expand or move in the opposite direction, and then give two specific update rules of the coefficient for Lasso and ridge regression. Moreover, we analyze the convergence properties of our algorithms for strongly convex problems, which show that both of our algorithms attain linear convergence rates. We also provide the convergence guarantees of our algorithms for non-strongly convex problems. Our experimental results further verify that our algorithms achieve significantly better performance than their counterparts.Comment: 25 pages, 8 figure

Memory and aging effects in interacting sub-10nm nanomagnets with large uniaxial anisotropy

Using a nonequilibrium Monte Carlo method suitable to nanomagnetism, we investigate representative systems of interacting sub-10nm grained nanomagnets with large uniaxial anisotropy. Various magnetization memory and aging effects are found in such systems. We explain these dynamical effects using the distributed relaxation times of the interacting nanomagnets due to their large anisotropy energies.Comment: Physics Letters A, in pres