NETLab@IISc

We are broadly interested in studying the thermal and electronic properties of semiconductors and metals. We develop computational and experimental tools to probe the microscopic, quantum-mechanical interactions among energy carriers that drive the macroscopic properties like thermal conductivity, electrical conductivity, electronic mobility and thermoelectric coefficients. A description of our published work and areas of current and future interest is given below:

Experimental Work

Thermal Transport and Phonon Spectroscopy using Pump-Probe Techniques

Phonons are quantized collective excitations of a crystal lattice, which are the primary heat carriers in semiconductors and electrical insulators. We study phonon thermal transport in crystalline solids using non-contact laser-based pump-probe techniques.

One such experiment is the transient grating, where a pair of coherent pump laser pulses form an instantaneous sinusoidal thermal excitation on the sample. The temporal decay of this thermal excitation is measured by a low-power probe laser, from which the thermal conductivity of the sample can be obtained. This experiment is well-suited to measure both in-plane and cross-plane thermal conductivity of most materials, and does not suffer from parasitic losses due to contact and interface resistances.

An exciting feature of the transient grating in our lab is that we can measure non-diffusive heat conduction phenomena, where the conventional Fourier's law of heat diffusion no longer holds. The non-diffusive heat conduction regime occurs when the period of the sinusoidal pump excitation, which is the lengthscale of heat transport in this experiment, is systematically reduced to become comparable to, or smaller than, the phonon mean free path (the average distance travelled by phonons before undergoing collision or scattering). In fact, we use the transition from the Fourier-diffusive to non-diffusive heat flow regimes to actually extract phonon-specific mean free paths experimentally.

Measurement of the fraction of phonons that undergo specular reflection at a rough surface using transient grating.
Navaneetha K. Ravichandran & Austin Minnich, Phys. Rev. B 93, 035314, 2016
Navaneetha K. Ravichandran, Hang Zhang & Austin Minnich, Phys. Rev. X 8 (4), 041004, 2018

We are setting up the transient grating experiment in our lab, and we will update the website shortly with more pictures!
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Sample Fabrication and Characterization

We perform thermal transport and phonon spectroscopy measurements on bulk crystals as well as suspended sub-micron thick membranes, sometimes with patterned geometries etched through them. We fabricate our membranes using wet-etch techniques, starting from a device-on-insulator crystal platform, as described in Navaneeth's Ph.D. thesis. The suspended thin films fabricated by this technique have good yield and surface quality/roughness characteristics, as seen from the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images.

Characterization of the surface quality and roughness on a suspended sub-micron thick silicon membrane (fabricated using wet-etch technique) using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Navaneetha K. Ravichandran, Ph.D. thesis, California Institute of Technology (Caltech), 2016

Computational Work

First-principles Prediction of the Thermal Conductivity of Semiconductors and Electrical Insulators

We have developed a unified first principles computational framework, based on density functional theory and the semi-classical Boltzmann transport equation, to predict the thermal conductivity of any semiconducting or electrically-insulating crystalline material, without any adjustable parameters. This framework accurately captures the resistance to heat flow caused by the scattering processes that phonons undergo, as they transport heat through the crystal.

While a large body of literature exists on the computational treatment of the lowest-order scattering processes involving three phonons at a time, our code is one of the first to rigorously compute the effect of higher-order scattering among four phonons on the thermal conductivity of materials. Using this advance, in 2018, we accurately predicted the ultrahigh thermal conductivity of a new material that is not available in nature - boron arsenide (BAs), which agreed well with the measurements from our experimental collaborators. Most importantly, we found that the inclusion of four-phonon scattering processes in our calculations is crucial to obtain quantitative agreement with the experimental data.

Subsequently, in 2020, we predicted the ultrahigh thermal conductivity of cubic boron nitride (BN) rivalling that of diamond, and a giant effect of isotopic mass disorder on its thermal conductivity as well. We also showed that four phonon scattering processes sharply increase the resistance to heat flow in several other group III-V semiconductors (e.g., InP, AlSb and BSb).

The insights gained from these studies drive our current and future computational efforts towards searching for new ultrahigh thermal conductivity candidates.

[1] Fei Tian, Bai Song, Xi Chen, Navaneetha K. Ravichandran et al.,Science 361(6402), 2018
[2] Ke Chen†, Bai Song†, Navaneetha K. Ravichandran† et al.,Science 367(6477), 2020 [†: equal contribution]
[3] Navaneetha K. Ravichandran & David Broido, PRX 10, 021063, 2020

Tuning Thermal Conductivity with Pressure

In most materials, application of hydrostatic pressure results in a linearly increasing thermal conductivity. However, we predict that for two of the ultrahigh thermal conductivity materials - BAs and boron phosphide (BP), the thermal conductivity shows a rare non-monotonic pressure dependence. Our microscopic first principles calculations show that this novel pressure-dependence of the thermal conductivity of BP and BAs are driven by exquisite competitions among different phonon scattering channels. Our calculations also show that BP shows one of the steepest pressure dependencies of thermal conductivity among all materials, which results in a two-fold enhancement of its thermal conductivity over an applied pressure of about 50 GPa.

The discovery of such novel needle in a haystack phenomena are only possible due to the predictive nature of our first principles computational framework, which does not require any adjustmentable parameters. We continue to look for novel thermal transport phenomena in other unexplored materials using our in-house code.

[1] Navaneetha K. Ravichandran & David Broido, Nature Communications 10(827), 2019
[2] Navaneetha K. Ravichandran & David Broido, Nature Communications 12(3473), 2021

First-principles Prediction of the Thermodynamic Properties of Materials

Our unified first principles framework has also been proven to predict the thermodynamic properties of crystalline solids. By accurately calculating higher-order anharmonic corrections to the Helmholtz free energy of the crystal, we have obtained excellent quantitative agreement for the coefficient of lattice thermal expansion (obtained from the temperature- and volume-derivatives of the Helmoltz free energy) with experiments on even strongly anharmonic materials like sodium chloride crystal (NaCl). To achieve such high accuracy, we have developed and implemented a self-consistent phonon renormalization scheme, which also captures the experimentally observed unusual temperature-dependence of the phonons in NaCl as well.

Our ability to accurately capture the phonons and the anharmonic free energy of a crystal, particularly for strongly anharmonic systems, opens the possibility of predicting material properties under extreme environments, such as very close to phase transitions or under extreme temperatures and pressures.

First principles prediction of the rate of lattice thermal expansion and the temperature dependence of phonons in a strongly anharmonic material - NaCl, using a newly-developed self-consistent phonon renormalization framework.
Navaneetha K. Ravichandran & David Broido, Phys. Rev. B 98, 085205, 2018

Thermal Transport through Sub-micron Thick Membranes & Nanostructures

The first principles calculations described above are applicable for infinitely large, periodic, pristine single crystals. For nanostructures and nanoscale devices, these calculations are prohibitively expensive. To overcome this computational complexity, we have developed a hybrid Monte Carlo solution of the Boltzmann equation for phonon transport, directly in the nanostructure geometry, by incorporating the first principles phonon properties as inputs.

This computationally efficient variance-reduced Monte Carlo code enables us to compute resistances to heat flow due to boundaries, interfaces and extended defects such as grain boundaries in nanoscale devices.

Prediction of the thermal conductivity of sub-micron thick membranes and nanostructures with lithographic patterns, using a variance-reduced computationally efficient Monte-Carlo solution of the Boltzmann equation for phonon transport.
Navaneetha K. Ravichandran & Austin Minnich, Phys. Rev. B 89, 205432, 2014