My name is Solomon F. Duki. I was born and raised in Addis Ababa, Ethiopia, the sixth child of my parents, among ten siblings. I am a theoretical condensed matter physicist by training, where I got my degrees from Addis Ababa University, Ethiopia (BSc, MSc), the Abdus Salam International Center for Theoretical Physics, Trieste, Italy (Diploma) and Case Western Reserve University, in Cleveland (MA, Ph.D.). Currently working as a researcher at the National Institute of Health (NIH) - National Center for Biotechnology Information (NCBI).

As a child, I have always been interested in math and science. I developed a great deal of interest in physics when I was a 10th-grade student. I give credit to two people who inspired me in this endeavor. One is my older brother Endeshaw (also a physicist) and another is my then 10th-grade physics teacher, Amare Hailu.

As a child, I have always been interested in math and science. I developed a great deal of interest in physics when I was a 10th-grade student. I give credit to two people who inspired me in this endeavor. One is my older brother Endeshaw (also a physicist) and another is my then 10th-grade physics teacher, Amare Hailu.

Normally the tenth-grade curriculum at the time was only on the basics of Electricity and Magnetism for the entire year. However, Amare’s class included special sessions on some advanced topics that he taught in between, without any detailed math, from modern physics and special theory of relativity. These include time dilation and length contraction; wave nature of particles, Compton scattering, and concepts like discrete energy (without mentioning any quantization). By the end of my 10th grade, I knew I had already developed a passion for physics to pursue it as a career.

My first research paper was from my master’s thesis at AAU, where we studied the non-equilibrium dynamics of Brownian particles in the presence of a position-dependent temperature background. This was one of the classic problems that were first suggested by Landauer (the blow-torch problem) where he argued that local dynamics matter for entropy production at the macroscopic level. In our work, we analyze the relaxation behavior of a bistable system when the background temperature profile is inhomogeneous due to the presence of a localized hot region (blowtorch) on one side of the potential barrier. Since the diffusion equation for the inhomogeneous medium is model-dependent, we considered two physical models to study the kinetics of such a system. We found the similarities and differences of the escape rates and, hence, exposes the common and distinct features of the two known physical models in determining the way the bistable system relaxes. Though my first work, this paper is being cited more, as we were the first to confirm that the blowtorch effect on the escape and equilibration rates of such a system are model-dependent.

In my graduate studies at Case Western, I first worked on strongly correlated systems mainly involved in the physics of low dimensional and low temperature effects in different systems. These include the tunneling and Fano resonance of electrons on the surface of liquid helium, generation of Hawking radiation in superfluid helium, charge fractionalization in one and two dimensional structures, Fano resonance in photonic crystals, Kondo resonance in one and two dimensional structures that have novel symmetries in their spin and orbital sectors and quantum adiabatic approximation.

To give you brief descriptions of my thesis work on the correlated system I pick two of my works; one is a problem of electrons on the surface of liquid helium in the presence of a perpendicular magnetic field. In this system, we applied a small in-plane magnetic field to study Fano resonance. Certain states that were bound to the helium surface then dissolve into the continuum turning into long-lived resonances. As a result, microwave absorption lines acquire an asymmetric Fano line- shape that is tunable by varying the microwave polarization or the in-plane magnetic field. Electrons trapped in a formerly bound state will tunnel off the surface of helium; we show that under suitable circumstances this “radioactive decay” can show damped oscillations rather than a simple exponential decay. This non-exponential oscillatory decay is remarkable in that its mechanism is not specific to electrons on helium and hence such effect may also be relevant elsewhere in physics.

Another one is a single channel (one dimensional) Kondo Model where the impurity spin is replaced by a general SU(n) spin. Using Bosonization and canonical transformation, we explicitly shown that such a system has an exactly solvable point and the solvable point is universal for all values of integer n.

In the later part of my graduate studies, I worked more on modeling and simulations of soft matter systems where we devised new techniques by which the glass transition temperature of any polymer can be predicted with minimal computational effort. To test our hypothesis we studied several polymers using atomistic molecular dynamics simulations and the mean squared displacements of their molecules have been analyzed by our new techniques. These techniques, which utilize the convoluted-velocity autocorrelation and the curvature of the mean squared displacement, efficiently predict the glass transition temperature of the polymers from short-time simulations.

Building on what I learned in soft matter systems in my graduate school I applied for a postdoctoral position at the University of Pittsburgh to develop the model for viscoelastic nanogels, and that of synthetic cilia using Lattice Boltzmann methods. One of the biggest projects I worked on at Pittsburgh was that I developed a new model to simulated a self-healing material composed of dually crosslinked nanogels. In such materials, permanently crosslinked nanogels particles are bound together through two kinds of cross-links, namely, the stable and labile ones. Under sufficiently high stress, the strong, stable bonds undergo irreversible rupture, whereas the weak, labile bonds can reform after breakage. We demonstrate that the presence of the labile inter-particle bonds makes possible the structural rearrangements inside the deformed material. As a result, the catastrophic failure of the material is postponed, and the defects (cavities) in the strained material heal themselves when the stress is released. We developed a mathematical model used in the simulations through a bottom-up approach that utilizes finite element methods in the lattice-spring model and it captures the viscoelastic properties of the material under various deformation regimes.

Following my postdoctoral experience, I took a two-year visiting assistant professor position at the Department of Physics & Astronomy at Rowan University in New Jersey. While teaching different undergraduate courses at Rowan, I continued to work on modeling and simulations of soft matter systems by collaborating with my colleagues at Rowan and the University of Akron.

One of the studies (with Rowan group) is in the designing of protein alloy materials for Biomedical applications. Using Lattice Spring Model I modeled and simulated Resilin, the type of elastic proteins found in the flight and jumping systems of most insects. These proteins are perfect super rubber with an elastic efficiency of more than 95% under high-frequency motion. Incredibly, they could be stretched over 300% of their original size with a low-elastic modulus of 0.1-3 MPa. Since the network in resilin is formed by crosslinking of tyrosine-residues as di- and trityrosine complexes we modeled them as cross-linked nanogels and predicted the correct mechanical and elastic behavior of the system.

Another project I was involved while I was at Rowan is the one I worked with my long-time mentor (since my high school days 30 years ago!) and colleague at the University of Akron, Mesfin Tsige. The problem we worked was on the studies of the phase behavior of water at low temperature and high pressure. Motivated by an experimental finding on the density of supercooled water at high pressure we performed atomistic molecular dynamics simulations study of bulk water in the isothermal-isobaric ensemble. By cooling and heating the water cyclically (at different isobars and isothermal compression) for a range of pressure, we were able to pinpoint a strong concave down curvature observation between the temperatures 180 K and 220 K. However, below the glass transition temperature, which is around 180 K at 1GPa, the volume turns to concave upward curvature. Since there was no crystallization was observed for the supercooled liquid state below 180 K, even after running the system for a very long time, our work strongly supported the existence of the long-held view of the ‘no-man land zone’ of supercooled water!

Accepting the chance to get back to the physics of low dimensional correlated systems; in the fall of 2013 I joined the quantitative molecular biological physics group (QMBP) at NCBI/NIH as an IRTA fellow to work on quantum magnetism. At QMBP I have been working on quantum spin-chain models to understand magnetic sensing in animals. The idea that animals can detect the magnetic field has traveled a long way from being fiction to a well-established fact in the last six decades. A lot of experimental evidence has now shown that many migratory animals, such as birds, whales, sea turtles, etc. use the detection of the earth's magnetic field to sense the direction of their migrations. In spite of the clear experimental evidence, however, the biological mechanisms of magnetic sensing are poorly understood. Our theoretical work is to shed light on this mechanism through quantum magnetism.

In this regard, one of the problems we studied was to understand the robustness of the Hund’s rule in a spin chain system to force higher spin moments in the low energy excitation spectrum. For example, quantum spin chains with composite spins have been used to approximate conventional chains with higher spin moments. This is to say, for instance, a chain with two spin 1/2’s per site can sometimes approximate a spin 1 chain. However, there was little examination given as to whether this approximation, effectively assuming the first Hund rule per site, is valid and why it works. Our work investigated the validity of this approximation through numerical diagonalization of the Hamiltonians (of spin chains with a spin 1 per site and with two spin 1/2’s per site). The low energy excitation spectrum for chains with two spin 1/2's per site is found to coincide with that of the corresponding conventional spin 1 chain, with one spin 1 per site. It turns out that as the system size increases, an increasingly larger block of consecutive lowest energy states with maximal spin per site is observed, robustly supporting the first Hund rule even though the exclusion principle does not apply, and the system does not possess Coulomb repulsion. As for why this approximation works, we show that this effective Hund rule emerges as a plausible consequence when applying to composite spin systems the Lieb–Mattis theorem, which was first elucidated for the ground state of ferromagnetic and antiferromagnetic spin systems. Using the “*Nit-string*” methods we developed to handle more degrees of freedom per site for large spin chain size, we are currently exploring many other features of antiferromagnetic systems using composite spin ½ model.

At the same time, I have also been working with another long-time a colleague of mine, Mesfin Taye, on the non-equilibrium noise-induced thermally activated stochastic processes to study first arrival time for excitable systems and stochastic resonance. In these models, we explored a thermally activated barrier crossing rate of a Brownian particle system in the presence of time-varying signals. We explored the dependence of signal to noise ratio (SNR) as well as the power amplification (η) on model parameters. In the presence of N particles, η is considerably amplified as N steps up showing the weak periodic signal, which plays a vital role in controlling the noise-induced dynamics of the system, a model that gives a crucial step to understand the intracellular calcium dynamics in a cardiac system.

To wrap it up, I have enjoyed working on a variety of problems in soft and hard condensed matter physics, by collaborating with different colleagues (see some of them in this links of my research papers and specific chapters of books). What gets me jazzed about science and research is the joy of solving physical problems and learning new methods in the process, which is a great deal of my life. If I weren’t doing what I am doing now, I might have pursued a career in pure mathematics, which is my second passion. Aside from research, I enjoy reading history books and spend more time with my family, enjoy playing basketball with my son (and watch him as he gets better at it), watching my daughter swims at a higher level, and run my long-distance course while everyone is sleeping!

My first research paper was from my master’s thesis at AAU, where we studied the non-equilibrium dynamics of Brownian particles in the presence of a position-dependent temperature background. This was one of the classic problems that were first suggested by Landauer (the blow-torch problem) where he argued that local dynamics matter for entropy production at the macroscopic level. In our work, we analyze the relaxation behavior of a bistable system when the background temperature profile is inhomogeneous due to the presence of a localized hot region (blowtorch) on one side of the potential barrier. Since the diffusion equation for the inhomogeneous medium is model-dependent, we considered two physical models to study the kinetics of such a system. We found the similarities and differences of the escape rates and, hence, exposes the common and distinct features of the two known physical models in determining the way the bistable system relaxes. Though my first work, this paper is being cited more, as we were the first to confirm that the blowtorch effect on the escape and equilibration rates of such a system are model-dependent.

In my graduate studies at Case Western, I first worked on strongly correlated systems mainly involved in the physics of low dimensional and low temperature effects in different systems. These include the tunneling and Fano resonance of electrons on the surface of liquid helium, generation of Hawking radiation in superfluid helium, charge fractionalization in one and two dimensional structures, Fano resonance in photonic crystals, Kondo resonance in one and two dimensional structures that have novel symmetries in their spin and orbital sectors and quantum adiabatic approximation.

To give you brief descriptions of my thesis work on the correlated system I pick two of my works; one is a problem of electrons on the surface of liquid helium in the presence of a perpendicular magnetic field. In this system, we applied a small in-plane magnetic field to study Fano resonance. Certain states that were bound to the helium surface then dissolve into the continuum turning into long-lived resonances. As a result, microwave absorption lines acquire an asymmetric Fano line- shape that is tunable by varying the microwave polarization or the in-plane magnetic field. Electrons trapped in a formerly bound state will tunnel off the surface of helium; we show that under suitable circumstances this “radioactive decay” can show damped oscillations rather than a simple exponential decay. This non-exponential oscillatory decay is remarkable in that its mechanism is not specific to electrons on helium and hence such effect may also be relevant elsewhere in physics.

Another one is a single channel (one dimensional) Kondo Model where the impurity spin is replaced by a general SU(n) spin. Using Bosonization and canonical transformation, we explicitly shown that such a system has an exactly solvable point and the solvable point is universal for all values of integer n.

In the later part of my graduate studies, I worked more on modeling and simulations of soft matter systems where we devised new techniques by which the glass transition temperature of any polymer can be predicted with minimal computational effort. To test our hypothesis we studied several polymers using atomistic molecular dynamics simulations and the mean squared displacements of their molecules have been analyzed by our new techniques. These techniques, which utilize the convoluted-velocity autocorrelation and the curvature of the mean squared displacement, efficiently predict the glass transition temperature of the polymers from short-time simulations.

Building on what I learned in soft matter systems in my graduate school I applied for a postdoctoral position at the University of Pittsburgh to develop the model for viscoelastic nanogels, and that of synthetic cilia using Lattice Boltzmann methods. One of the biggest projects I worked on at Pittsburgh was that I developed a new model to simulated a self-healing material composed of dually crosslinked nanogels. In such materials, permanently crosslinked nanogels particles are bound together through two kinds of cross-links, namely, the stable and labile ones. Under sufficiently high stress, the strong, stable bonds undergo irreversible rupture, whereas the weak, labile bonds can reform after breakage. We demonstrate that the presence of the labile inter-particle bonds makes possible the structural rearrangements inside the deformed material. As a result, the catastrophic failure of the material is postponed, and the defects (cavities) in the strained material heal themselves when the stress is released. We developed a mathematical model used in the simulations through a bottom-up approach that utilizes finite element methods in the lattice-spring model and it captures the viscoelastic properties of the material under various deformation regimes.

Following my postdoctoral experience, I took a two-year visiting assistant professor position at the Department of Physics & Astronomy at Rowan University in New Jersey. While teaching different undergraduate courses at Rowan, I continued to work on modeling and simulations of soft matter systems by collaborating with my colleagues at Rowan and the University of Akron.

One of the studies (with Rowan group) is in the designing of protein alloy materials for Biomedical applications. Using Lattice Spring Model I modeled and simulated Resilin, the type of elastic proteins found in the flight and jumping systems of most insects. These proteins are perfect super rubber with an elastic efficiency of more than 95% under high-frequency motion. Incredibly, they could be stretched over 300% of their original size with a low-elastic modulus of 0.1-3 MPa. Since the network in resilin is formed by crosslinking of tyrosine-residues as di- and trityrosine complexes we modeled them as cross-linked nanogels and predicted the correct mechanical and elastic behavior of the system.

Another project I was involved while I was at Rowan is the one I worked with my long-time mentor (since my high school days 30 years ago!) and colleague at the University of Akron, Mesfin Tsige. The problem we worked was on the studies of the phase behavior of water at low temperature and high pressure. Motivated by an experimental finding on the density of supercooled water at high pressure we performed atomistic molecular dynamics simulations study of bulk water in the isothermal-isobaric ensemble. By cooling and heating the water cyclically (at different isobars and isothermal compression) for a range of pressure, we were able to pinpoint a strong concave down curvature observation between the temperatures 180 K and 220 K. However, below the glass transition temperature, which is around 180 K at 1GPa, the volume turns to concave upward curvature. Since there was no crystallization was observed for the supercooled liquid state below 180 K, even after running the system for a very long time, our work strongly supported the existence of the long-held view of the ‘no-man land zone’ of supercooled water!

Accepting the chance to get back to the physics of low dimensional correlated systems; in the fall of 2013 I joined the quantitative molecular biological physics group (QMBP) at NCBI/NIH as an IRTA fellow to work on quantum magnetism. At QMBP I have been working on quantum spin-chain models to understand magnetic sensing in animals. The idea that animals can detect the magnetic field has traveled a long way from being fiction to a well-established fact in the last six decades. A lot of experimental evidence has now shown that many migratory animals, such as birds, whales, sea turtles, etc. use the detection of the earth's magnetic field to sense the direction of their migrations. In spite of the clear experimental evidence, however, the biological mechanisms of magnetic sensing are poorly understood. Our theoretical work is to shed light on this mechanism through quantum magnetism.

In this regard, one of the problems we studied was to understand the robustness of the Hund’s rule in a spin chain system to force higher spin moments in the low energy excitation spectrum. For example, quantum spin chains with composite spins have been used to approximate conventional chains with higher spin moments. This is to say, for instance, a chain with two spin 1/2’s per site can sometimes approximate a spin 1 chain. However, there was little examination given as to whether this approximation, effectively assuming the first Hund rule per site, is valid and why it works. Our work investigated the validity of this approximation through numerical diagonalization of the Hamiltonians (of spin chains with a spin 1 per site and with two spin 1/2’s per site). The low energy excitation spectrum for chains with two spin 1/2's per site is found to coincide with that of the corresponding conventional spin 1 chain, with one spin 1 per site. It turns out that as the system size increases, an increasingly larger block of consecutive lowest energy states with maximal spin per site is observed, robustly supporting the first Hund rule even though the exclusion principle does not apply, and the system does not possess Coulomb repulsion. As for why this approximation works, we show that this effective Hund rule emerges as a plausible consequence when applying to composite spin systems the Lieb–Mattis theorem, which was first elucidated for the ground state of ferromagnetic and antiferromagnetic spin systems. Using the “

At the same time, I have also been working with another long-time a colleague of mine, Mesfin Taye, on the non-equilibrium noise-induced thermally activated stochastic processes to study first arrival time for excitable systems and stochastic resonance. In these models, we explored a thermally activated barrier crossing rate of a Brownian particle system in the presence of time-varying signals. We explored the dependence of signal to noise ratio (SNR) as well as the power amplification (η) on model parameters. In the presence of N particles, η is considerably amplified as N steps up showing the weak periodic signal, which plays a vital role in controlling the noise-induced dynamics of the system, a model that gives a crucial step to understand the intracellular calcium dynamics in a cardiac system.

To wrap it up, I have enjoyed working on a variety of problems in soft and hard condensed matter physics, by collaborating with different colleagues (see some of them in this links of my research papers and specific chapters of books). What gets me jazzed about science and research is the joy of solving physical problems and learning new methods in the process, which is a great deal of my life. If I weren’t doing what I am doing now, I might have pursued a career in pure mathematics, which is my second passion. Aside from research, I enjoy reading history books and spend more time with my family, enjoy playing basketball with my son (and watch him as he gets better at it), watching my daughter swims at a higher level, and run my long-distance course while everyone is sleeping!

© The Ethiopian Physics Society in North America. All rights reserved.

Support Us