arXiv subjects

David G. Cahill

Publications and source records attributed to David G. Cahill.

At least 19 recordsLinked to original sources

arXiv2025

Strong Temperature Dependence of Thermal Conductivity in High-Purity Cubic Boron Arsenide

Materials with high thermal conductivity are needed to conduct heat away from hot spots in high power electronics and optoelectronic devices. Cubic boron arsenide (c-BAs) has a high thermal conductivity due to its special phonon dispersion relation. Previous experimental studies of c-BAs report a room-temperature thermal conductivity between 1000 and 1300 W m-1 K-1. We synthesized high purity isotopically enriched c-BAs single crystals with room-temperature thermal conductivity of around 1500 W m-1 K-1. Using time-domain thermoreflectance (TDTR), we measured thermal conductivity and found a 1/T2 temperature dependence between 300 K and 600 K - slightly stronger than predictions from state-of-the-art theoretical models. Brillouin and Raman scattering revealed minimal changes in phonon frequencies over the same temperature range, suggesting that the observed 1/T2 dependence is not caused by temperature dependent changes in phonon dispersion. To probe defect densities in the BAs crystals we studied, we conducted transient reflectivity microscopy (TRM) measurements of absorption at sub-bandgap photon energies. We observe a correlation between TRM signal intensity and thermal conductivity. Notably, samples with thermal conductivity near 1500 W m-1 K-1 still exhibited nonzero TRM signals, suggesting the presence of defects despite the high thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2024

Thermal contribution to current-driven antiferromagnetic-order switching

In information technology devices, current-driven state switching is crucial in various disciplines including spintronics, where the contribution of heating to the switching mechanism plays an inevitable role. Recently, current-driven antiferromagnetic order switching has attracted considerable attention due to its implications for next-generation spintronic devices. Although the switching mechanisms can be explained by spin dynamics induced by spin torques, some reports have claimed that demagnetization above the Neel temperature due to Joule heating is critical for switching. Here we present a systematic method and an analytical model to quantify the thermal contribution due to Joule heating in micro-electronic devices, focusing on current-driven octupole switching in the non-collinear antiferromagnet, Mn3Sn. The results consistently show that the critical temperature for switching remains relatively constant above the Neel temperature, while the threshold current density depends on the choice of substrate and the base temperature. In addition, we provide an analytical model to calculate the Joule-heating temperature which quantitatively explains our experimental results. From numerical calculations, we illustrate the reconfiguration of magnetic orders during cooling from a demagnetized state of polycrystalline Mn3Sn. This work not only provides deeper insights into magnetization switching in antiferromagnets, but also a general guideline for evaluating the Joule-heating temperature excursions in micro-electronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Improving nuclear magnetic resonance and electron spin resonance thermometry with size reduction of superparamagnetic iron oxide nanoparticles

Thermometry based on magnetic resonance has been extensively studied due to its important application in biomedical imaging. In our previous work, we showed that the spin-spin relaxation time (T2) of nuclear magnetic resonance (NMR) in water is a highly sensitive thermometer as T2 scales with the highly temperature-sensitive self-diffusion constant of water. In this work, in addition to temperature dependent self-diffusion constant of a fluid, we utilize the temperature dependent magnetization of 4 nm SPIONs to improve T2 sensitivity (4.96) by 1.4 times over self-diffusion (3.48) alone in hexane between 248 K and 333 K. To extend the application of NMR T2 thermometry to engineering systems, we also investigate the temperature dependence of T2 in mineral oil (Thermo Scientific, J62592), which exhibits remarkably high sensitivity (11.62) between 273 K and 353 K. This result implies that applications of NMR T2 thermometry in heat transfer fluids are promising. NMR thermometry, however, is generally not applicable to solids. Therefore, we also evaluate the potential of electron spin resonance (ESR) thermometry with SPIONs in solids between 100 K and 290 K, for potential temperature monitoring in biomedical and engineering applications. The size and concentration effects on ESR signals are studied systematically, and our results show that the temperature dependent linewidth follows a T^-2 law for 4 nm SPIONs, while the concentration of SPIONs has no impact on the temperature dependence of the ESR linewidth. The linewidth at room temperature at 9.4 GHz is 10.5 mT. Combining our NMR and ESR results, we find that to obtain higher temperature sensitivity in a magnetic resonance technique using SPIONs, SPION with a small magnetic moment, i.e., a small volume and reduced magnetization, are beneficial.

physics.app-ph↗

arXiv2024

Temperature-dependent optical and magneto-optical spectra of ferromagnetic BCC Fe

Optical and magneto-optical properties of magnetic materials have been widely exploited to characterize magnetic structures and phenomena, however, their temperature dependence is not well understood. This study implements the supercell approach with thermal lattice and magnetic disorders to obtain optical and magneto-optical spectra at finite temperatures based on Williams-Lax theory. Our results show that large optical spectrum signals are generated at photon energies below 1 eV, originating from the phonon- and magnon-assisted intraband transitions as lattice and magnetic temperatures increase. In addition, the prominent peak near 2.7 eV is redshifted proportionally to magnetic temperature but depends much less on lattice temperature. By analyzing unfolded bands, we show that the reduction of exchange splitting due to the thermal demagnetization causes this redshift. Our unfolded electronic band structure with magnetic disorder shows band kinks, which are characteristic evidence of the coupling between electrons and magnetic excitations. First-order magneto-optical spectra at finite temperature are also predicted, but due to their small magnitude suffer more from sampling errors. We discuss the effect of zero-point vibrations and the connection of these simulations to the Drude model for intraband transitions.

cond-mat.mtrl-sci↗

arXiv2023

Ionic Peltier Effect in Li-Ion Electrolytes

The coupled transport of charge and heat provide fundamental insights into the microscopic thermodynamics and kinetics of materials. We describe a sensitive ac differential resistance bridge that enables measurements of the temperature difference on two sides of a coin cell with a resolution of better than 10 uK. We use this temperature difference metrology to determine the ionic Peltier coefficients of symmetric Li-ion electrochemical cells as a function of Li salt concentration, solvent composition, electrode material, and temperature. The Peltier coefficients Π are negative, i.e., heat flows in the direction opposite to the drift of Li ions in the applied electric field, large, 30 kJ mol-1, and increase with increasing temperature at T > 300 K. The Peltier coefficient is approximately constant on time scales that span the characteristic time for mass diffusion across the thickness of the electrolyte, suggesting that heat of transport plays a minor role in comparison to the changes in partial molar entropy of Li at the interface between the electrode and electrolyte. Our work demonstrates a new platform for studying the non-equilibrium thermodynamics of electrochemical cells and provides a window into the transport properties of electrochemical materials through measurements of temperature differences and heat currents that complement traditional measurements of voltages and charge currents.

cond-mat.mtrl-sci↗

arXiv2023

Anisotropic thermal conductivity of high bandwidth memory

Thermal management of integrated circuits (ICs) is important to prevent thermal hotspots which are the leading cause of IC failure. Thermal management is even more critical in 3D integrated circuits (3D ICs) as the prevalence of thermal hotspots is expected to increase due to the presence of polymers and solder materials that are of low thermal conductivity. Understanding how thermal conductivity is affected by the presence of these materials is required for developing thermally aware IC design. The 3ω method can measure thermal conductivities spanning several orders of magnitude and is appropriate for measuring the thermal properties of layered structures such as 3D ICs. In this work, we use the 3ω method with planar and cylindrical heat flow geometries to determine thermal conductivities of the memory layers and layers with polymer and solder bumps in High Bandwidth Memory (HBM) Random Access Memory (RAM). We determine the in-plane thermal conductivity of the memory layers in HBM as 140 W/m-K, while the through-plane conductivity of the polymer/solder bump layer is 2 W/m-K. Combining the results of x-ray tomography and the 3-omega measurements, we estimate that the effective in-plane thermal conductivity of the overall HBM device is 100 W/m-K while the effective through-plane thermal conductivity is 7 W/m-K. Our results show that the presence of polymers and solder metals results in a significantly decreased through-plane thermal conductivity of a 3D IC compared to a single IC die. Improvement in the thermal performance of 3D ICs will require improvement in the thermal conductivity or the increased contact area of the solder metals used in 3D ICs.

physics.app-ph↗

arXiv2023

Temperature dependence of 7Li NMR relaxation rates in Li3InCl6, Li3YCl6, Li1.48Al0.48Ge1.52(PO4)3 and LiPS5Cl

Inorganic solid-state battery electrolytes show high ionic conductivities and enable the fabrication of all solid-state batteries. In this work, we present the temperature dependence of spin-lattice relaxation time (T1), spin-spin relaxation time (T2), and resonance linewidth of the 7Li nuclear magnetic resonance (NMR) for four solid-state battery electrolytes (Li3InCl6 (LIC), Li3YCl6 (LYC), Li1.48Al0.48Ge1.52(PO4)3 (LAGP) and LiPS5Cl (LPSC)) from 173 K to 403 K at a 7Li resonance frequency of 233 MHz, and from 253 K to 353 K at a 7Li resonance frequency of 291 MHz. Additionally, we measured the spin-lattice relaxation rates at an effective 7Li resonance frequency of 133 kHz using a spin-locking pulse sequence in the temperature range of 253 K to 353 K. In LPSC, the 7Li NMR relaxation is consistent with the Bloembergen-Pound-Purcell (BPP) theory of NMR relaxation of dipolar nuclei. In LIC, LYC and LAGP, the BPP theory does not describe the NMR relaxation rates for the temperature range and frequencies of our measurements. The presented NMR relaxation data assists in providing a complete picture of Li diffusion in the four solid-state battery electrolytes.

cond-mat.mtrl-sci↗

arXiv2023

Electron paramagnetic resonance of n-type silicon and germanium for applications in 3D thermometry

While several 2D thermometry techniques exist, there is a lack of 3D thermometry techniques that work for wide range of materials and offer good resolution in time, space and temperature. X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) imaging can provide 3D temperature information. However, XRD is typically limited to crystalline materials while NMR is largely limited to liquids where the resonance lines are narrow. We investigate electron paramagnetic resonance (EPR) of n-type silicon and germanium for 3D thermometry. While in germanium the EPR linewidths are too broad, EPR linewidths in silicon are reasonably narrow and exhibit a strong temperature dependence. The temperature dependence of the spin-lattice relaxation rate (1/T1) of conduction electrons in n-type Si for low dopant concentrations follows a T^3 law due to phonon broadening. For heavily doped Si, which is desirable for good signal to noise ratio (SNR) for application in thermometry, impurity scattering is expected to decrease the temperature dependence of 1/T1. Our results show, in heavily doped n-type Si, spin-lattice relaxation induced by impurity scattering does not drastically decrease the temperature dependence of EPR linewidths. In P-doped Si with donor concentration of 7 x 10^18 /cm^3, the EPR linewidth has a T^(5/2) temperature dependence; the temperature dependence decreases to T^(3/2) when the donor concentration is 7 x 10^19 /cm^3. While the temperature dependence of linewidth decreases for heavier doping, EPR linewidth is still a sensitive thermometer. We define a figure of merit for SNR for thermometry from EPR linewidths of n-type Si and observe that increasing the doping results in a better SNR. Using effective medium theory, we show that EPR linewidth can be a sensitive thermometer for application in 3D thermometry with systems embedding microparticles of heavily doped n-type Si.

cond-mat.mtrl-sci↗

arXiv2023

Simultaneous mapping of temperature and hydration in proton exchange membrane of fuel cells using magnetic resonance imaging

The efficiency of a proton exchange membrane (PEM) fuel cell depends on the mobility of protons in the PEM, which is determined by the hydration and temperature of the membrane. While optical techniques or neutron or x-ray scattering techniques may be used to study the inhomogeneities in hydration and temperature in PEMs, these techniques cannot provide 3 dimensional spatial resolution in measuring layered PEMs. Due to their ability to provide non-invasive 3D images, spin-lattice relaxation time (T1) and spin-spin relaxation time (T2) contrast magnetic resonance imaging (MRI) of protons in PEMs have been suggested as methods to map hydration in the fuel cells. We show that while T1 and T2 imaging may be used to map hydration in PEMs under isothermal conditions, proton T1 and T2 are also a function of temperature. For PEM fuel cells, where current densities are large and thermal gradients are expected, T1 and T2 relaxation times cannot be used for mapping hydration. The chemical shift of the mobile proton is, however, a strong function of hydration but not temperature. Therefore, chemical shift imaging (CSI) can be used to map hydration. The diffusion constant of the mobile proton, which can be determined by pulsed field gradient NMR, increases with both temperature and hydration. Therefore, CSI followed by imaging of diffusion via pulsed field gradients can be used for separate mappings of hydration and temperature in PEMs. Here, we demonstrate a 16 x 16 pixel MRI mapping of hydration and temperature in Nafion PEMs with a spatial resolution of 1 mm x 1 mm, a total scan time of 3 minutes, a temperature resolution of 6 K, and an uncertainty in hydration within 15%. The demonstrated mapping can be generalized for imaging exchange membranes of any fuel cells or flow batteries.

cond-mat.mtrl-sci↗

arXiv2023

Highly sensitive and high throughput magnetic resonance thermometry using superparamagnetic nanoparticles

Magnetic resonance imaging (MRI) enables non-invasive 3D thermometry during thermal ablation of cancerous tumors. While T1 or T2 contrast MRI are relatively insensitive to temperature, techniques with greater temperature sensitivity such as chemical shift or diffusion imaging suffer from motional artifacts and long scan times. We describe an approach for highly sensitive and high throughput MR thermometry that is not susceptible to motional artifacts. We use superparamagnetic iron oxide nanoparticles (SPIONs) to spoil T2 of water protons. Motional narrowing results in proportionality between T2 and the diffusion constant, dependent only on the temperature in a specific environment. Our results show, for pure water, the nuclear magnetic resonance (NMR) linewidth and T2 follow the same temperature dependence as the self-diffusion constant of water. Thus, T2 mapping is a diffusion mapping in the presence of SPIONs, and T2 is a thermometer. For pure water, a T2 mapping of a 64 x 64 image (voxel size = 0.5 mm x 0.5 mm x 3 mm) in a 9.4 T MRI scanner resulted in a temperature resolution of 0.5 K for a scan time of 2 minutes. This indicates a highly sensitive and high throughput MR thermometry technique potentially useful for monitoring of biological tissues during thermal therapies or for diagnosis.

physics.med-ph↗

arXiv2022

Angstrom-Scale Imaging of Magnetization in Antiferromagnetic Fe$_2$As via 4D-STEM

We demonstrate a combination of computational tools and experimental 4D-STEM methods to image the local magnetic moment in antiferromagnetic Fe$_2$As with 6 angstrom spatial resolution. Our techniques utilize magnetic diffraction peaks, common in antiferromagnetic materials, to create imaging modes that directly visualize the magnetic lattice. Using this approach, we show that center-of-mass analysis can determine the local magnetization component in the plane perpendicular to the path of the electron beam. Moreover, we develop Magnstem, a quantum mechanical electron scattering simulation code, to model electron scattering of an angstrom-scale probe from magnetic materials. Using these tools, we identify optimal experimental conditions for separating weak magnetic signals from the much stronger interactions of an angstrom-scale probe with electrostatic potentials. Our techniques should be useful for characterizing the local magnetic order in systems such in thin films, interfaces, and domain boundaries of antiferromagnetic materials, which are difficult to probe with existing methods.

cond-mat.mtrl-sci↗

arXiv2022

Frequency-domain probe beam deflection method for measurement of thermal conductivity of materials on micron length scale

Time-domain thermoreflectance (TDTR) and frequency-domain thermoreflectance (FDTR) have been widely used for non-contact measurement of anisotropic thermal conductivity of materials with high spatial resolution. However, the requirement of high thermoreflectance coefficient restricts the choice of metal coating and laser wavelength. The accuracy of the measurement is often limited by the high sensitivity to the radii of the laser beams. We describe an alternative frequency-domain pump-probe technique based on probe beam deflection. The beam deflection is primarily caused by thermoelastic deformation of the sample surface with a magnitude determined by the thermal expansion coefficient of the bulk material to measure. We derive an analytical solution to the coupled elasticity and heat diffusion equations for periodic heating of a multilayer sample with anisotropic elastic constants, thermal conductivity, and thermal expansion coefficients. In most cases, a simplified model can reliably describe the frequency dependence of the beam deflection signal without knowledge of the elastic constants and thermal expansion coefficients of the material. The magnitude of the probe beam deflection signal is larger than the maximum magnitude achievable by thermoreflectance detection of surface temperatures if the thermal expansion coefficient is greater than 5x10^(-6) /K. The sensitivity to laser beam radii is suppressed when a larger beam offset is used. We find nearly perfect matching of the measured signal and model prediction, and measure thermal conductivities within 6% of accepted values for materials spanning the range of polymers to gold, 0.1 - 300 W/(m K).

physics.app-ph↗

arXiv2022

Topological Metal MoP Nanowire for Interconnect

The increasing resistance of Cu interconnects for decreasing dimensions is a major challenge in continued downscaling of integrated circuits beyond the 7-nm technology node as it leads to unacceptable signal delays and power consumption in computing. The resistivity of Cu increases due to electron scattering at surfaces and grain boundaries of the interconnects at the nanoscale. Topological semimetals, owing to their topologically protected surface states and suppressed electron backscattering, are promising material candidates to potentially replace current Cu interconnects as low-resistance interconnects. Here, we report the attractive resistivity scaling of topological metal MoP nanowires and show that the resistivity values are comparable to those of Cu interconnects below 500 nm$^2$ cross-section areas. More importantly, we demonstrate that the dimensional scaling of MoP nanowires, in terms of line resistance versus total cross-sectional area, is superior to those of effective Cu and barrier-less Ru interconnects, suggesting MoP is an attractive solution to the current scaling challenge of Cu interconnects.

cond-mat.mtrl-sci↗

arXiv2022

High Thermal Conductivity in Wafer Scale Cubic Silicon Carbide Crystals

High thermal conductivity electronic materials are critical components for high-performance electronic and photonic devices as either active functional materials or thermal management materials. We report an isotropic high thermal conductivity over 500 W m-1K-1 at room temperature in high-quality wafer-scale cubic silicon carbide (3C-SiC) crystals, which is the second highest among large crystals (only surpassed by diamond). Furthermore, the corresponding 3C-SiC thin films are found to have record-high in-plane and cross-plane thermal conductivity, even higher than diamond thin films with equivalent thicknesses. Our results resolve a long-lasting puzzle that the literature values of thermal conductivity for 3C-SiC are perplexingly lower than the structurally more complex 6H-SiC. Further analysis reveals that the observed high thermal conductivity in this work arises from the high purity and high crystal quality of 3C-SiC crystals which excludes the exceptionally strong defect-phonon scatterings in 3C-SiC. Moreover, by integrating 3C-SiC with other semiconductors by epitaxial growth, we show that the measured 3C-SiC-Si TBC is among the highest for semiconductor interfaces. These findings not only provide insights for fundamental phonon transport mechanisms, also suggest that 3C-SiC may constitute an excellent wide-bandgap semiconductor for applications of power electronics as either active components or substrates.

cond-mat.mtrl-sci↗

arXiv2022

Temperature mapping of stacked silicon dies from x-ray diffraction intensities

Increasing power densities in integrated circuits has led to an increased prevalence of thermal hotspots in integrated circuits. Tracking these thermal hotspots is imperative to prevent circuit failures. In 3D integrated circuits, conventional surface techniques like infrared thermometry are unable to measure 3D temperature distribution and optical and magnetic resonance techniques are difficult to apply due to the presence of metals and large current densities. X-rays offer high penetration depth and can be used to probe 3D structures. We report a method utilizing the temperature dependence of x-rays diffraction intensity via the Debye-Waller factor to simultaneously map the temperature of an individual silicon die that is a part of a stack of dies. Utilizing beamline 1-ID-E at the Advanced Photon Source (Argonne), we demonstrate for each individual silicon die, a temperature resolution of 3 K, a spatial resolution of 100 um x 400 um and a temporal resolution of 20 s. Utilizing a sufficiently high intensity laboratory source, e.g., from a liquid anode source, this method can be scaled down to laboratories for non-invasive temperature mapping of 3D integrated circuits.

physics.app-ph↗

arXiv2021

Phonon, Electron, and Magnon Excitations in Antiferromagnetic L1$_{0}$-type MnPt

Antiferromagnetic L1$_{0}$-type MnPt is a material with relatively simple crystal and magnetic structure, recently attracting interest due to its high N{é}el temperature and wide usage as a pinning layer in magnetic devices. While it is experimentally well characterized, the theoretical understanding is much less developed, in part due to the challenging accuracy requirements dictated by the small underlying energy scales that govern magnetic ordering in antiferromagnetic metals. In this work, we use density functional theory, the Korringa-Kohn-Rostoker formalism, and a Heisenberg model to establish a comprehensive theoretical description of antiferromagnetic L1$_{0}$-type MnPt, along with accuracy limits, by thoroughly comparing to available literature data. Our simulations show that the contribution of the magnetic dipole interaction to the magnetocrystalline anisotropy energy of $K_{1}$=1.07$\times 10^{6}$\,J/m$^3$ is comparable in magnitude to the spin-orbit contribution. Using our result for the magnetic susceptibility of $5.25\times10^{-4}$, a lowest magnon frequency of about 2.02\,THz is predicted, confirming THz spin dynamics in this material. From our data for electron, phonon, and magnon dispersion we compute the individual contributions to the total heat capacity and show that the dominant term at or above 2\,K arises from phonons. From the Landau-Lifshitz-Gilbert equation, we compute a Néel temperature of 990--1070 K. Finally, we quantify the magnitude of the magneto-optical Kerr effect generated by applying an external magnetic field. Our results provide insight into the underlying physics, which is critical for a deep understanding of fundamental limits of the time scale of spin dynamics, stability of the magnetic ordering, and the possibility of magneto-optical detection of collective spin motion.

cond-mat.mtrl-sci↗

arXiv2021

Thermal conductivity of intercalation, conversion, and alloying lithium-ion battery electrode materials as function of their state of charge

Upon insertion and extraction of lithium, materials important for electrochemical energy storage can undergo changes in thermal conductivity ($Λ$) and elastic modulus ($\it M$). These changes are attributed to evolution of the intrinsic thermal carrier lifetime and interatomic bonding strength associated with structural transitions of electrode materials with varying degrees of reversibility. Using in situ time-domain thermoreflectance (TDTR) and picosecond acoustics, we systemically study $Λ$ and $\it M$ of conversion, intercalation and alloying electrode materials during cycling. The intercalation V$_{2}$O$_{5}$ and TiO$_{2}$ exhibit non-monotonic reversible $Λ$ and $\it M$ switching up to a factor of 1.8 ($Λ$) and 1.5 ($\it M$) as a function of lithium content. The conversion Fe$_{2}$O$_{3}$ and NiO undergo irreversible decays in $Λ$ and $\it M$ upon the first lithiation. The alloying Sb shows the largest and partially reversible order of the magnitude switching in $Λ$ between the delithiated (18 W m$^{-1}$ K$^{-1}$) and lithiated states (<1 W m$^{-1}$ K$^{-1}$). The irreversible $Λ$ is attributed to structural degradation and pulverization resulting from substantial volume changes during cycling. These findings provide new understandings of the thermal and mechanical property evolution of electrode materials during cycling of importance for battery design, and also point to pathways for forming materials with thermally switchable properties.

physics.app-ph↗

arXiv2021

Direct Observation of Reversible Heat Absorption in Li-ion Battery Enabled by Ultra-Sensitive Thermometry

The reversible heat in lithium-ion batteries (LIBs) due to entropy change is fundamentally important for understanding the chemical reactions in LIBs and developing proper thermal management strategies. However, the direct measurements of reversible heat are challenging due to the limited temperature resolution of applied thermometry. In this work, by developing an ultra-sensitive thermometry with a differential AC bridge using two thermistors, the noise-equivalent temperature resolution we achieve (10 uK) is several orders of magnitude higher than previous thermometry applied on LIBs. We directly observe reversible heat absorption of a LIR2032 coin cell during charging with negligible irreversible heat generation and a linear relation between heat generations and discharging currents. The cell entropy changes determined from the reversible heat agree excellently with those measured from temperature dependent open circuit voltage. Moreover, it is found that the large reversible entropy change can cancel out the irreversible entropy generation at a charging rate as large as C/3.7 and produce a zero-heat-dissipation LIB during charging. Our work significantly contributes to fundamental understanding of the entropy changes and heat generations of the chemical reactions in LIBs, and reveals that reversible heat absorption can be an effective way to cool LIBs during charging.

physics.app-ph↗