A Study on Performance and Power Efficiency of Dense Non-Volatile Caches in Multi-Core Systems

In this paper, we present a novel cache design based on Multi-Level Cell Spin-Transfer Torque RAM (MLC STTRAM) that can dynamically adapt the set capacity and associativity to use efficiently the full potential of MLC STTRAM. We exploit the asymmetric nature of the MLC storage scheme to build cache lines featuring heterogeneous performances, that is, half of the cache lines are read-friendly, while the other is write-friendly. Furthermore, we propose to opportunistically deactivate ways in underutilized sets to convert MLC to Single-Level Cell (SLC) mode, which features overall better performance and lifetime. Our ultimate goal is to build a cache architecture that combines the capacity advantages of MLC and performance/energy advantages of SLC. Our experiments show an improvement of 43% in total numbers of conflict misses, 27% in memory access latency, 12% in system performance, and 26% in LLC access energy, with a slight degradation in cache lifetime (about 7%) compared to an SLC cache

Data-Driven Modal Analysis of Turbulent Momentum Exchange and Heat Transfer in Composite Porous Fluid Systems

This paper investigates the dynamics governing turbulent momentum exchange and heat transfer between pore flow within porous media and the turbulent flow passing over it. Employing high-fidelity pore-scale large eddy simulation, our investigation explores the fundamental mechanisms driving these phenomena. Modal analysis based on snapshot Proper Orthogonal Decomposition (POD) is employed to quantify the modes of interaction between porous and non-porous regions, providing a comprehensive understanding of the underlying processes. Spatial and temporal modes reveal the existence of localized flow structures at the pore scale, contributing to time-varying patterns of information exchange. At the commencement of the porous block, the mean flow (Mode = 0) from the porous to the non-porous region is the dominant mechanism in momentum exchange and heat transfer. This mode facilitates convective heat transfer from the porous to the non-porous region through upward and forward flow movements, showcasing positive flow leakage. In addition to the mean flow, the turbulent flux inherent in alternate POD modes (Mode ≠ 0) plays a substantial role in information propagation, influencing diverse directions. Spatial modes, complemented by statistical analysis, uncover a significant likelihood of observing negative vertical velocity values in the wake of the porous ligaments at the porous-fluid interface, indicative of negative flow leakage. This negative flow leakage precisely corresponds to the local penetration of fluid from the non-porous region into the porous region. Furthermore, our study reveals that information exchange via turbulence fluctuations manifests through complex outward and inward interactions in regions characterized by substantial positive flow leakage. Notably, these regions exhibit a distinct tendency for high-momentum streamwise-oriented flow to migrate outward from the porous region into the non-porous region (outward interactions). Conversely, inward interactions arise in these regions when the instantaneous magnitude of positive flow leakage is smaller than the mean value of positive flow leakage, emphasizing the pulsating nature of positive flow leakage. Finally, the distribution of the Nusselt number highlights that more than 60% of total heat transfer occurs within the initial one-third of the porous block length. Significantly, a notable portion of the porous ligaments experiences insufficient cooling due to positive flow leakage, underlining the critical implications of these findings for the understanding of turbulent momentum exchange and heat transfer in a composite porous-fluid system

Plasmonic and Ultrafast Optical Response of 2D and 3D Dirac Materials

The fast-evolving field of condensed matter physics is witnessing a rapid development of a new class of materials, called Dirac materials. The low-energy electronic excitation in these materials behaves like massless Dirac particles. These materials exhibit unique optoelectronic properties, and understanding of Dirac quasi-particle dynamics in two and three dimensions is imperative to realizing the potential applications. In this dissertation, we study two prominent Dirac materials that have unique optoelectronic properties: graphene (two-dimensional) and tantalum arsenide (three-dimensional). While the former can be regarded as the father of materials with a symmetry-protected Dirac spectrum, the latter is a more recent example of topology-protected Dirac materials, also known as 3D Weyl semimetals. We employ spectroscopy and ultrafast optical techniques to study plasmons, and the interaction/relaxation dynamics of photo-excited carriers in these materials. More specifically, we study a new class of plasmon resonances in hybrid metal-graphene structures, which is an important step towards practical graphene plasmonic optoelectronic devices. In addition, we investigate the giant nonlinear THz response of graphene plasmons using pump-probe techniques and discuss the physical origin of the plasmon-enhanced nonlinearity. Furthermore, we introduce a novel continuous-wave photomixing spectroscopy technique to investigate the frequency dependence and nonlinearity of hot-electron cooling in graphene. Finally, we explore the relaxation dynamics of photo-excited Weyl fermions in tantalum arsenide via ultrafast optical pump-probe techniques, which shed light on the electron-phonon relaxation processes in this material

Pore-scale Conjugate Heat Transfer Analysis of Turbulent Flow over Stochastic Open-cell Metal Foams

Fundamental understanding of turbulent flow and heat transfer in composite porous-fluid systems, which consists of a fluid-saturated stochastic open-cell metal foam and a flow passing over it, is crucial for fostering technological development in numerous applications such as transpiration cooling in aerospace, packed-bed thermal storage and thermal management of electronic devices. In this work, conjugate heat transfer simulations were adopted to explore the turbulent flow and heat transfer features in a composite porous-fluid system at the pore-scale. Simulations were performed to account for the influence of the blockage ratios (i.e., BR = 0.5, 0.8 and 1.0) on pressure drop and heat transfer rate by introducing a new concept called penetration cooling length. Furthermore, the effect of Reynolds numbers (i.e., Re = 1800, 3600 and 7200) at different blockage ratios was investigated in terms of pressure drop, fluid and solid temperatures, interstitial heat transfer coefficient, and flow leakage. Results indicate that for a fixed blockage ratio, as the Reynolds number increases by a factor of 3.0, there is a 14.9-fold increase in the pressure drop and a 2.9-fold increase in the interstitial heat transfer coefficient. Additionally, for a fixed Reynolds number, when the blockage ratio increases by a factor of 2.0, there is a 6.8-fold increase in the pressure drop and a 1.8-fold increase in the interstitial heat transfer coefficient. Flow visualisation indicated that the penetration cooling length is influenced by flow leakage from the porous-fluid interface. A correlation of IHTC is proposed based Reynolds number, blockage ratio and development length of the metal foam. Results show at small blockage ratios and low Reynolds numbers, a significant portion of the flow from the porous region leaves it to the clear region on top of the porous block. While, at high Reynolds numbers and large blockage ratios, the flow leakage is reduced. Additionally, for a low blockage ratio (BR<0.5), the amount of flow leakage depends on the Reynolds number, while it is independent of the Reynolds number for BR>0.8