Bjorn Vermeersch

Bio: Bjorn Vermeersch is an academic researcher from Purdue University. The author has contributed to research in topics: Thermal resistance & Thermal conductivity. The author has an hindex of 19, co-authored 68 publications receiving 1162 citations. Previous affiliations of Bjorn Vermeersch include University of Grenoble & University of California, Santa Cruz.

Temperature and Thickness Dependences of the Anisotropic In‐Plane Thermal Conductivity of Black Phosphorus

Abstract: The anisotropic basal-plane thermal conductivities of thin black phosphorus obtained from a new four-probe measurement exhibit much higher peak values at low temperatures than previous reports. First principles calculations reveal the important role of crystal defects and weak thickness dependence that is opposite to the case of graphene and graphite due to the absence of reflection symmetry in puckered phosphorene.

Superdiffusive heat conduction in semiconductor alloys. I. Theoretical foundations

Abstract: Semiconductor alloys exhibit a strong dependence of effective thermal conductivity on measurement frequency. So far this quasiballistic behavior has only been interpreted phenomenologically, providing limited insight into the underlying thermal transport dynamics. Here, we show that quasiballistic heat conduction in semiconductor alloys is governed by L\'evy superdiffusion. By solving the Boltzmann transport equation (BTE) with ab initio phonon dispersions and scattering rates, we reveal a transport regime with fractal space dimension $1l\ensuremath{\alpha}l2$ and superlinear time evolution of mean-square energy displacement ${\ensuremath{\sigma}}^{2}(t)\ensuremath{\sim}{t}^{\ensuremath{\beta}}(1l\ensuremath{\beta}l2)$. The characteristic exponents are directly interconnected with the order $n$ of the dominant phonon scattering mechanism $\ensuremath{\tau}\ensuremath{\sim}{\ensuremath{\omega}}^{\ensuremath{-}n}(ng3)$ and cumulative conductivity spectra ${\ensuremath{\kappa}}_{\ensuremath{\Sigma}}(\ensuremath{\tau};\ensuremath{\Lambda})\ensuremath{\sim}{(\ensuremath{\tau};\ensuremath{\Lambda})}^{\ensuremath{\gamma}}$ resolved for relaxation times or mean free paths through the simple relations $\ensuremath{\alpha}=3\ensuremath{-}\ensuremath{\beta}=1+3/n=2\ensuremath{-}\ensuremath{\gamma}$. The quasiballistic transport inside alloys is no longer governed by Brownian motion, but instead is dominated by L\'evy dynamics. This has important implications for the interpretation of thermoreflectance (TR) measurements with modified Fourier theory. Experimental $\ensuremath{\alpha}$ values for InGaAs and SiGe, determined through TR analysis with a novel L\'evy heat formalism, match ab initio BTE predictions within a few percent. Our findings lead to a deeper and more accurate quantitative understanding of the physics of nanoscale heat-flow experiments.

Superdiffusive heat conduction in semiconductor alloys. II. Truncated Lévy formalism for experimental analysis

Abstract: Nearly all experimental observations of quasiballistic heat flow are interpreted using Fourier theory with modified thermal conductivity. Detailed Boltzmann transport equation (BTE) analysis, however, reveals that the quasi-ballistic motion of thermal energy in semiconductor alloys is no longer Brownian but instead exhibits L\'evy dynamics with fractal dimension $\ensuremath{\alpha}l2$. Here, we present a framework that enables full three-dimensional experimental analysis by retaining all essential physics of the quasiballistic BTE dynamics phenomenologically. A stochastic process with just two fitting parameters describes the transition from pure L\'evy superdiffusion as short length and time scales to regular Fourier diffusion. The model provides accurate fits to time domain thermoreflectance raw experimental data over the full modulation frequency range without requiring any ``effective'' thermal parameters and without any a priori knowledge of microscopic phonon scattering mechanisms. Identified $\ensuremath{\alpha}$ values for InGaAs and SiGe match ab initio BTE predictions within a few percent. Our results provide experimental evidence of fractal L\'evy heat conduction in semiconductor alloys. The formalism additionally indicates that the transient temperature inside the material differs significantly from Fourier theory and can lead to improved thermal characterization of nanoscale devices and material interfaces.

Full-field thermal imaging of quasiballistic crosstalk reduction in nanoscale devices

Abstract: Understanding nanoscale thermal transport is of substantial importance for designing contemporary semiconductor technologies. Heat removal from small sources is well established to be severely impeded compared to diffusive predictions due to the ballistic nature of the dominant heat carriers. Experimental observations are commonly interpreted through a reduction of effective thermal conductivity, even though most measurements only probe a single aggregate thermal metric. Here, we employ thermoreflectance thermal imaging to directly visualise the 2D temperature field produced by localised heat sources on InGaAs with characteristic widths down to 100 nm. Besides displaying effective thermal performance reductions up to 50% at the active junctions in agreement with prior studies, our steady-state thermal images reveal that, remarkably, 1–3 μm adjacent to submicron devices the crosstalk is actually reduced by up to fourfold. Submicrosecond transient imaging additionally shows responses to be faster than conventionally predicted. A possible explanation based on hydrodynamic heat transport, and some open questions, are discussed. When thermal fields in semiconductors approach the submicron scale, non-diffusive heat transport is observed where Fourier based heat transport models fail. Here, the authors use thermal imaging to visualise these thermal field variations and in turn derive a hydrodynamic heat transport model.

L\'evy walks

Abstract: Random walk is a fundamental concept with applications ranging from quantum physics to econometrics. Remarkably, one specific model of random walks appears to be ubiquitous across many fields as a tool to analyze transport phenomena in which the dispersal process is faster than dictated by Brownian diffusion. The Levy walk model combines two key features, the ability to generate anomalously fast diffusion and a finite velocity of a random walker. Recent results in optics, Hamiltonian chaos, cold atom dynamics, bio-physics, and behavioral science demonstrate that this particular type of random walks provides significant insight into complex transport phenomena. This review provides a self-consistent introduction to Levy walks, surveys their existing applications, including latest advances, and outlines further perspectives.

Metal-Ion-Modified Black Phosphorus with Enhanced Stability and Transistor Performance.

Abstract: Black phosphorus (BP), a burgeoning elemental 2D semiconductor, has aroused increasing scientific and technological interest, especially as a channel material in field-effect transistors (FETs). However, the intrinsic instability of BP causes practical concern and the transistor performance must also be improved. Here, the use of metal-ion modification to enhance both the stability and transistor performance of BP sheets is described. Ag+ spontaneously adsorbed on the BP surface via cation-π interactions passivates the lone-pair electrons of P thereby rendering BP more stable in air. Consequently, the Ag+ -modified BP FET shows greatly enhanced hole mobility from 796 to 1666 cm2 V-1 s-1 and ON/OFF ratio from 5.9 × 104 to 2.6 × 106 . The mechanisms pertaining to the enhanced stability and transistor performance are discussed and the strategy can be extended to other metal ions such as Fe3+ , Mg2+ , and Hg2+ . Such stable and high-performance BP transistors are crucial to electronic and optoelectronic devices. The stability and semiconducting properties of BP sheets can be enhanced tremendously by this novel strategy.

Blueprint for a microwave trapped ion quantum computer

Abstract: The availability of a universal quantum computer may have a fundamental impact on a vast number of research fields and on society as a whole. An increasingly large scientific and industrial community is working toward the realization of such a device. An arbitrarily large quantum computer may best be constructed using a modular approach. We present a blueprint for a trapped ion–based scalable quantum computer module, making it possible to create a scalable quantum computer architecture based on long-wavelength radiation quantum gates. The modules control all operations as stand-alone units, are constructed using silicon microfabrication techniques, and are within reach of current technology. To perform the required quantum computations, the modules make use of long-wavelength radiation–based quantum gate technology. To scale this microwave quantum computer architecture to a large size, we present a fully scalable design that makes use of ion transport between different modules, thereby allowing arbitrarily many modules to be connected to construct a large-scale device. A high error–threshold surface error correction code can be implemented in the proposed architecture to execute fault-tolerant operations. With appropriate adjustments, the proposed modules are also suitable for alternative trapped ion quantum computer architectures, such as schemes using photonic interconnects.