arXiv subjects

Xi Luo

Publications and source records attributed to Xi Luo.

At least 55 records · Page 3Linked to original sources

arXiv2019

Anomalous magnetic transport and extra quantum oscillation in semi-metallic photon-like fermion gas

In the absence of Lorentz symmetry, the pseudospin-1 counterpart of Weyl fermion (feroton) with linear dispersions and an exact flat band can emerge in condensed matter systems. The flat band branch of feroton is equivalent to the longitudinal photon in Maxwell theory, which is a redundant degree of freedom due to the emergent (fermionic) gauge symmetry. Upon coupling to an external magnetic field, the fermionic guage symmetry is broken and the flat band ferotons become gapless excitations characterized by Landau level indices ($n>1$). In the long wave length limit, these gapless modes are of the opposite chirality to the chiral anomaly related zero Landau level, which leads to much more plentiful magnetic transport properties. To further explore the novel properties of these gapless modes, we investigate the quantum oscillation through a generalized Lieb lattice model. We find an extra oscillating behavior which indicates the existence of these exotic gapless modes. We collect known {\it ab initio} calculation data from literature and discuss the possibility of realizing the semi-metallic feroton gas in real materials.

cond-mat.mes-hall↗

arXiv2018

Model Assisted Variable Clustering: Minimax-optimal Recovery and Algorithms

Model-based clustering defines population level clusters relative to a model that embeds notions of similarity. Algorithms tailored to such models yield estimated clusters with a clear statistical interpretation. We take this view here and introduce the class of G-block covariance models as a background model for variable clustering. In such models, two variables in a cluster are deemed similar if they have similar associations will all other variables. This can arise, for instance, when groups of variables are noise corrupted versions of the same latent factor. We quantify the difficulty of clustering data generated from a G-block covariance model in terms of cluster proximity, measured with respect to two related, but different, cluster separation metrics. We derive minimax cluster separation thresholds, which are the metric values below which no algorithm can recover the model-defined clusters exactly, and show that they are different for the two metrics. We therefore develop two algorithms, COD and PECOK, tailored to G-block covariance models, and study their minimax-optimality with respect to each metric. Of independent interest is the fact that the analysis of the PECOK algorithm, which is based on a corrected convex relaxation of the popular K-means algorithm, provides the first statistical analysis of such algorithms for variable clustering. Additionally, we contrast our methods with another popular clustering method, spectral clustering, specialized to variable clustering, and show that ensuring exact cluster recovery via this method requires clusters to have a higher separation, relative to the minimax threshold. Extensive simulation studies, as well as our data analyses, confirm the applicability of our approach.

stat.ME↗

arXiv2018

Functional Mediation Analysis with an Application to Functional Magnetic Resonance Imaging Data

Causal mediation analysis is widely utilized to separate the causal effect of treatment into its direct effect on the outcome and its indirect effect through an intermediate variable (the mediator). In this study we introduce a functional mediation analysis framework in which the three key variables, the treatment, mediator, and outcome, are all continuous functions. With functional measures, causal assumptions and interpretations are not immediately well-defined. Motivated by a functional magnetic resonance imaging (fMRI) study, we propose two functional mediation models based on the influence of the mediator: (1) a concurrent mediation model and (2) a historical mediation model. We further discuss causal assumptions, and elucidate causal interpretations. Our proposed models enable the estimation of individual causal effect curves, where both the direct and indirect effects vary across time. Applied to a task-based fMRI study, we illustrate how our functional mediation framework provides a new perspective for studying dynamic brain connectivity. The R package cfma is available on CRAN.

stat.AP↗

arXiv2017

Majorana fermions in three dimensions and realization in critical Weyl semimetals

We present two band models for free fermion with charge conjugation symmetry in three dimensions. Without time reversal symmetry (TRS), the weak pairing gapless $A$-phase is a Majorana fermion $p_x+ip_y$ wave FFLO state while the strong pairing gapped $B$-phase belongs to topologically trivial Class $D$. With TRS, there is a Majorana fermion $B$-phase belonging to Class $DIII$ with a non-zero Hopf invariant. The TRS $A$-phase is also a Majorana fermion FFLO state with TRS. The surface states of the TRS $B$-phase are either a valley-momentum locked Majorana-Dirac cone or a linear-quadratic mixed cone for a specific surface. The surface states of the $A$-phase on one surface are topologically nontrivial, either having $\mathbb{Z}$ or $\mathbb{Z}_2$ invariant depending on whether the system is TRS or not. The edge states of that surface are gapless Majorana modes. The Majorana fermion gapless FFLO states can be realized in critical Weyl semimetals (WSM) in which dual single Weyl nodes form dipoles and are nearly annihilated. The gapped $B$-phase emerges when Weyl node dipoles are about to be created. The WSM TaAs-family, a type-II WSM series Mo$_x$W$_{1-x}$Te$_2$-family, possible WSM La/LuBi$_{1-x}$ Sb$_x$Te$_3$ and topological crystalline insulators Sn$_{1-x}$Pb$_x$(Te,Se) are candidates to be manipulated into these critical states based on Majorana fermion models.

cond-mat.mes-hall↗

arXiv2017

Granger Mediation Analysis of Multiple Time Series with an Application to fMRI

It becomes increasingly popular to perform mediation analysis for complex data from sophisticated experimental studies. In this paper, we present Granger Mediation Analysis (GMA), a new framework for causal mediation analysis of multiple time series. This framework is motivated by a functional magnetic resonance imaging (fMRI) experiment where we are interested in estimating the mediation effects between a randomized stimulus time series and brain activity time series from two brain regions. The stable unit treatment assumption for causal mediation analysis is thus unrealistic for this type of time series data. To address this challenge, our framework integrates two types of models: causal mediation analysis across the variables and vector autoregressive models across the temporal observations. We further extend this framework to handle multilevel data to address individual variability and correlated errors between the mediator and the outcome variables. These models not only provide valid causal mediation for time series data but also model the causal dynamics across time. We show that the modeling parameters in our models are identifiable, and we develop computationally efficient methods to maximize the likelihood-based optimization criteria. Simulation studies show that our method reduces the estimation bias and improve statistical power, compared to existing approaches. On a real fMRI data set, our approach not only infers the causal effects of brain pathways but accurately captures the feedback effect of the outcome region on the mediator region.

stat.ME↗

arXiv2017

Realization of Massive Relativistic Spin-3/2 Rarita-Schwinger Quasiparticle in Condensed Matter Systems

The spin-3/2 elementary particle, known as Rarita-Schwinger (RS) fermion, is described by a vector-spinor field ψ_{μα}, whose number of components is larger than its independent degrees of freedom (DOF). Thus the RS equations contain nontrivial constraints to eliminate the redundant DOF. Consequently the standard procedure adopted in realizing relativistic spin-1/2 quasi-particle is not capable of creating the RS fermion in condensed matter systems. In this work, we propose a generic method to construct a Hamiltonian which implicitly contains the RS constraints, thus includes the eigenstates and energy dispersions being exactly the same as those of RS equations. By implementing our 16X16 or 6X6 Hamiltonian, one can realize the 3 dimensional or 2 dimensional (2D) massive RS quasiparticles, respectively. In the non-relativistic limit, the 2D 6X6 Hamiltonian can be reduced to two 3X3 Hamiltonians which describe the positive and negative energy parts respectively. Due to the nontrivial constraints, this simplified 2D massive RS quasiparticle has an exotic property: it has vanishing orbital magnetic moment while its orbital magnetization is finite. Finally, we discuss the material realization of RS quasiparticle. Our study provides an opportunity to realize higher spin elementary fermions with constraints in condensed matter systems.

cond-mat.mes-hall↗

arXiv2017

Rugged HBT Class-C Power Amplifiers with Base-Emitter Clamping

The ruggedness of HBT Class-C power amplifiers was improved by adding an anti-parallel diode to the amplifier input to limit the negative swing of the base-emitter voltage. The improved amplifier could withstand 3:1 instead of 2:1 mismatch in CW operation, and 2.5:1 instead of 1.5:1 mismatch in pulse operation. In contrast to other approaches with emitter ballast, active feedback, or electrostatic discharge protection circuits, the present approach is simple to implement and has negligible impact on overall amplifier output power, gain or efficiency.

physics.app-ph↗

arXiv2017

Scaling and High-Frequency Performance of AlN/GaN HEMTs

Small- and large-signal RF characteristics were measured on AlN GaN HEMTs with 80-160 nm gate length and 100-300 μm width. Consistent with the literature, current-gain cut-off frequency and maximum frequency of oscillation were found to increase with inverse gate length and independent of gate width. For the first time, output power and efficiency were reported at the high end of Xband, and were comparable to the best reported at 2 GHz and insensitive to gate length or width. These results suggest that the AlN/GaN HEMTs can be further scaled for even higher frequency and higher power performance.

cond-mat.mes-hall↗

arXiv2017

Nonlinear Modeling of MEMS Fixed-Fixed beams

This dissertation presents a new coupled electro-mechanical model that is an improvement on the classical parallel-plate approximation. The model employs a hyperbolic function to account for the beam deformed shape and electrostatic field. Based on this, the model can accurately calculate the deflection of a fixed-fixed beam subjected to an applied voltage and the switch capacitance-voltage characteristics without using parallel-plate assumption. For model validation, the model solutions are compared with ANSYS finite element results and experimental data. It is found that the model works especially well in residual stress dominant and stretching dominant cases. The model shows that the nonlinear stretching can significantly increase the pull-in voltage and extend the beam maximum travel range. Based on the model, a graphene nanoelectromechanical systems (NEMS) resonator is simulated and the agreement with the experimental data is excellent. The proposed coupled hyperbolic model demonstrates its capacity to guide the design and optimization of both RF microelectromechanical system (MEMS) capacitive switches and NEMS devices.

physics.app-ph↗

arXiv2017

Intermodulation distortion of actuated MEMS capacitive switches

For the first time, intermodulation distortion of micro-electromechanical capacitive switches in the actuated state was analyzed both theoretically and experimentally. The distortion, although higher than that of switches in the suspended state, was found to decrease with increasing bias voltage but to depend weakly on modulation frequencies between 55 kHz and 1.1 MHz. This dependence could be explained by the orders-of-magnitude increase of the spring constant when the switches were actuated. Additionally, the analysis suggested that increasing the spring constant and decreasing the contact roughness could improve the linearity of actuated switches. These results are critical to micro-electromechanical capacitive switches used in tuners, filters, phase shifters, etc. where the linearity of both suspended and actuated states are critical.

physics.app-ph↗

arXiv2017

Estimating Causal Mediation Effects under Correlated Errors

Causal mediation analysis usually requires strong assumptions, such as ignorability of the mediator, which may not hold in many social and scientific studies. Motivated by a multilevel randomized treatment experiment using functional magnetic resonance imaging (fMRI), this paper proposes a multilevel causal mediation framework for data with hierarchically nested structure, and this framework provides valid inference even if structured unmeasured confounding for the mediator and outcome is present. For the first-level data, we propose a linear structural equation model for a continuous mediator and a continuous outcome, both of which may contain correlated additive errors. A likelihood-based approach is proposed to estimate the model coefficients. The analysis of our estimator characterizes the nonidentifiability issue due to the correlation parameter. To address the identifiability issue and model the variability in multilevel data, we propose to incorporate multiple first-level mediation models across different levels in a unified multilevel mediation framework. All the model coefficients are estimated simultaneously by our optimization algorithms, with innovation to estimate the unknown correlation parameter from data, instead of performing sensitivity analysis. Our asymptotic analysis shows that the correlation parameter is identifiable, and our estimates for the mediation effects are consistent with the parametric convergence rates. Using extensive simulated data and a real fMRI dataset, we demonstrate the improvement of our approaches over existing methods.

stat.AP↗

arXiv2017

Nonperturbative emergence of Dirac fermion in strongly correlated composite fermions of fractional quantum Hall effects

The classic composite fermion field theory (Ref. 1) builds up an excellent framework to uniformly study important physical objects and globally explain anomalous experimental phenomena in fractional quantum Hall physics while there are also inherent weaknesses. We present a nonperturbative emergent Dirac fermion theory from this strongly correlated composite fermion field theory, which overcomes these serious long-standing shortcomings. The particle-hole symmetry of Dirac equation resolves this particle-hole symmetry enigma in the composite fermion field theory. With the help of presented numerical data, we show that for main Jain's sequences of fractional quantum Hall effects, this emergent Dirac fermion theory is most likely nonperturbatively stable.

cond-mat.str-el↗

arXiv2017

Inference with interference between units in an fMRI experiment of motor inhibition

An experimental unit is an opportunity to randomly apply or withhold a treatment. There is interference between units if the application of the treatment to one unit may also affect other units. In cognitive neuroscience, a common form of experiment presents a sequence of stimuli or requests for cognitive activity at random to each experimental subject and measures biological aspects of brain activity that follow these requests. Each subject is then many experimental units, and interference between units within an experimental subject is likely, in part because the stimuli follow one another quickly and in part because human subjects learn or become experienced or primed or bored as the experiment proceeds. We use a recent fMRI experiment concerned with the inhibition of motor activity to illustrate and further develop recently proposed methodology for inference in the presence of interference. A simulation evaluates the power of competing procedures.

stat.ME↗

arXiv2016

A Point-process Response Model for Spike Trains from Single Neurons in Neural Circuits under Optogenetic Stimulation

Optogenetics is a new tool to study neuronal circuits that have been genetically modified to allow stimulation by flashes of light. We study recordings from single neurons within neural circuits under optogenetic stimulation. The data from these experiments present a statistical challenge of modeling a high frequency point process (neuronal spikes) while the input is another high frequency point process (light flashes). We further develop a generalized linear model approach to model the relationships between two point processes, employing additive point-process response functions. The resulting model, Point-process Responses for Optogenetics (PRO), provides explicit nonlinear transformations to link the input point process with the output one. Such response functions may provide important and interpretable scientific insights into the properties of the biophysical process that governs neural spiking in response to optogenetic stimulation. We validate and compare the PRO model using a real dataset and simulations, and our model yields a superior area-under-the- curve value as high as 93% for predicting every future spike. For our experiment on the recurrent layer V circuit in the prefrontal cortex, the PRO model provides evidence that neurons integrate their inputs in a sophisticated manner. Another use of the model is that it enables understanding how neural circuits are altered under various disease conditions and/or experimental conditions by comparing the PRO parameters.

stat.ME↗

arXiv2016

Fast and Adaptive Sparse Precision Matrix Estimation in High Dimensions

This paper proposes a new method for estimating sparse precision matrices in the high dimensional setting. It has been popular to study fast computation and adaptive procedures for this problem. We propose a novel approach, called Sparse Column-wise Inverse Operator, to address these two issues. We analyze an adaptive procedure based on cross validation, and establish its convergence rate under the Frobenius norm. The convergence rates under other matrix norms are also established. This method also enjoys the advantage of fast computation for large-scale problems, via a coordinate descent algorithm. Numerical merits are illustrated using both simulated and real datasets. In particular, it performs favorably on an HIV brain tissue dataset and an ADHD resting-state fMRI dataset.

stat.ME↗

arXiv2016

Gauge fermions with flat bands and anomalous transport via chiral modes from breaking gauge symmetry

The dispersionless longitudinal photon in Maxwell theory is thought of as a redundant degree of freedom due to the gauge symmetry. We find that when there exist exactly flat bands with zero energy in a condensed matter system, the fermion field may locally transform as a gauge field and the system possesses a gauge symmetry. As the longitudinal photon, the redundant degrees of freedom from the flat bands must be gauged away from the physical states. As an example, we study spinless fermions on a generalized Lieb lattice in three dimensions. The flat band of the longitudinal fermion induces a gauge symmetry. An external magnetic field breaks this gauge symmetry and emerges a bunch of non-topologically chiral modes. Combining these emergent chiral modes with the chiral anomaly mode which is of an opposite chirality, rich anomalous electric transport phenomena exhibit and are expected to be observed in Pd$_3$Bi$_2$S$_2$ and Ag$_3$Se$_2$Au.

cond-mat.mes-hall↗

arXiv2016

Nonlinear Quantum Hall effects in Rarita-Schwinger gas

Emergence of higher spin relativistic fermionic materials becomes a new favorite in the study of condensed matter physics. Massive Rarita-Schwinger 3/2-spinor was known owning very exotic properties, such as the superluminal fermionic modes and even being unstable in an external magnetic field. Due to the superluminal modes and the non-trivial constraints on the Rarita-Schwinger gas, we exposit anomalous properties of the Hall effects in (2+1)-dimensions which subvert the well-known quantum Hall paradigms. First, the Hall conductance of a pure Rarita-Schwinger gas is step-like but not plateau-quantized, instead of the linear dependence on the filling factor for a pure spin-1/2 Dirac gas. In reality, the Hall conductance of the Dirac gas is of quantized integer plateaus with the unit $\frac{e^2}h$ due to the localization away from the Landau level centers. If the general localization rule is applicable to the disordered Rarita-Schwinger gas, the Hall plateaus are also expected to appear but they are nonlinearly dependent on the Landau level index. Furthermore, there is a critical magnetic field beyond which higher Landau levels become unstable. This confines the filling factor of the system. We also show that the non-hermitness of the effective Hamiltonian is not crucial to the nonlinearity of the quantum Hall conductance.

cond-mat.quant-gas↗

arXiv2016

Hybrid Weyl Semimetal

We construct a tight-binding model realizing one pair of Weyl nodes and three distinct Weyl semimetals. In the type-I (type-II) Weyl semimetal, both nodes belong to type-I (type-II) Weyl nodes. In addition, there exists a novel type, dubbed "hybrid Weyl semimetal", in which one Weyl node is of type-I while the other is of type-II. For the hybrid Weyl semimetal, we further demonstrate the bulk Fermi surfaces and the topologically protected surface states, analyze the unique Landau level structure and quantum oscillation, and discuss the material realization.

cond-mat.mes-hall↗