Tag Archives: Ehsan Khatami

Physics Professor Khatami Publishes Latest Groundbreaking Research in ‘Science’

Ehsan Khatami is one of two San Jose State University faculty members selected as an Early Career Investigator Award winner in 2017-18. (Photo: James Tensuan, '15 Journalism)

Ehsan Khatami is one of two San Jose State University faculty members selected as an Early Career Investigator Award winner in 2017-18. (Photo: James Tensuan, ’15 Journalism)

San Jose State University Associate Professor of Physics and Astronomy Ehsan Khatami in collaboration with a group of professors from MIT and the MIT-Harvard Center for Ultracold Atoms published today in the journal Science their latest experimental discovery about conduction in a tiny system of atoms in a vacuum.

Khatami, who was granted early tenure and promotion to associate professor this year, received a funding from the National Science Foundation with colleague Sen Chiao, of the Meteorology Department to build the first high-performance computing cluster on campus. The equipment has proven essential to his research as well as the work of students and faculty in other disciplines that require big data analysis.

In his most recent article, Khatami and his colleagues discuss an experiment that is impossible to perform using real materials. They were able to focus on the movement of atoms’ intrinsic magnetic field, or “spin,” across a few microns without disturbing their charge arrangement (charge is another intrinsic property of atoms) as the first of its kind with a quantum system. The results shed light on the mostly unexplored spin transport property of models condensed matter scientists use to describe the unusual behavior of solids at very low temperatures.

Atoms are like small magnets, so applying a magnetic force pushes them around, here to the left (top left). Since these atoms repel each other, they cannot move if there are no empty sites (top middle). But the atomic “magnetic needles” are still free to move, with stronger magnets (red) diffusing to the left in the image, and weaker magnets (blue) having to make room and move to the right (bottom row). This so-called spin transport is resolved atom by atom in the cold atom quantum emulator.

Atoms are like small magnets, so applying a magnetic force pushes them around, here to the left (top left). Since these atoms repel each other, they cannot move if there are no empty sites (top middle). But the atomic “magnetic needles” are still free to move, with stronger magnets (red) diffusing to the left in the image, and weaker magnets (blue) having to make room and move to the right (bottom row). This so-called spin transport is resolved atom by atom in the cold atom quantum emulator.

Khatami’s research aims to help scientists understand how superconductivity works—a finding that could potentially pave the way for a room-temperature superconductor, which would improve transportation and data storage and make homes more energy efficient by creating materials that allow better use of electricity. That is, as electricity goes through a device such as a phone or laptop, none of the electronic components would heat up. Superconductivity is the property of zero electrical resistance in some substances at very low temperatures (<-135 degrees Celsius).

The experiment was carried out using 400 atoms cooled down to just a hair above absolute zero temperature (<-273 degrees Celsius). The atoms were manipulated to be two different types and to act as if they were electrons in a solid with two species of spin. The atoms were then trapped in a square box to see how they would respond when magnetic fields keeping one species on the left side and one species on the right side of the box were turned off. Scientists watched the process by using an electron gas microscope to measure the speed at which mixing takes place and deduce the “spin” current.

Khatami compares the box of atoms to a shallow pool of water – if there was a divider in the middle with clear water on one side and water dyed black on the other side when the divider is suddenly removed the water would mix together and turn gray. The two shades of water would be similar to the two spin species in the quantum experiment, with the behavior of the atoms governed by quantum mechanics.

To support the experiment, Khatami used more than 300,000 CPU hours on SJSU’s Spartan High-Performance Computer to solve the underlying theoretical model that was emulated in the experiment to support experimental observations.

“As exciting as these findings have been, there are still so many unanswered questions we can explore using similar setups,” he said. “For example, the dependence of spin transport on the temperature or the concentration of atoms in the box can be studied.”

Khatami received the SJSU 2017-18 Early Career Investigator Award and has offered insights into his research on the web series Physics Girl. He was featured in the Fall/Winter 2018 edition of Washington Square alumni magazine.

SJSU Professor Publishes ‘Intriguing’ Findings on Ultracold Atoms

In a paper published Sept. 29 in the journal of Science, experimentalists at Princeton, led by Prof. Waseem Bakr, and several theorists, including Ehsan Khatami, an assistant professor of physics and astronomy atSJSU, report their direct observation of an exotic magnetic phase of matter that could help explain how high-temperature superconductivity — the complete loss of resistance to electric flow— works.

In their experiment, Bakr and the group used lithium atoms cooled down to billionths of a degree above absolute zero (< -273 degrees Celsius), a temperature at which quantum mechanical effects dominate, and used lasers to trap atoms in a small region of space, only a few tens of micrometers across. They also used lasers to create a virtual 2D crystal, resembling an empty egg-tray, known as the optical lattice. An atomic microscope was then used to image atoms that were loaded on this lattice.

Researchers found that applying a large magnetic field — the effect that causes bar magnets to attract or repel each other — to these atoms causes their intrinsic magnetic fields to alternate in alignment in a checkerboard pattern while slightly leaning away from each other, a state termed “canted antiferromagnetism”.

The experiment is designed so that atoms can hop from one site to the neighboring sites of the “egg-tray”, while mostly avoiding each other on the same site. If we “look” at these particles at high temperatures, they have so much energy they will be moving around and bouncing off each other randomly. If the temperature is low, however, a completely different picture emerges. What we will see under the microscope would be exotic behaviors we are not used to through our everyday experiences with classical particles. Atoms start to “collaborate” to try to optimize the use of the little energy they have left.

The collaboration between atoms becomes a lot more fascinating when there are two types of them mixed in on the optical lattice, such as in the Princeton experiment. Each atom can be thought of as bar magnet that can point its north pole either up or down. With an equal population of “up” and “down” atoms, they settle into a situation where their alignment alternates from one site to the neighboring site at low temperatures. In the experiment carried out at Princeton, a magnetic field resulted in an imbalance in the population of atoms and caused them to settle instead into an unusual magnetic state in which the anti-alignment of ups and downs is pushed to the plane perpendicular to the magnetic field but canted slightly in the direction of the field.

This study is an important step towards better understanding electronic properties of solids. The system simulated in this study is a near perfect realization of a theoretical model known as the Fermi-Hubbard model, widely believed to have the ingredients for describing high-temperature superconductivity in copper-oxide materials known as cuprates. Understanding the underlying quantum mechanism driving exotic behaviors such as superconductivity or the magnetic state observed in this study can help us design better materials with specific properties we can harness in technology, energy and industry applications.

Khatami used a state-of-the-art numerical technique he had helped develop to obtain exact results for the Fermi-Hubbard model with parameters relevant to the experiment. Comparisons of numerical results with the experimental measurements was crucial in guiding the experiments and allowed the team to obtain an estimate for the system’s temperature, verify how the population imbalance changes the correlations in the system, and characterize the new phase of atoms using those correlations.

Similar experiments, albeit in the absence of a magnetic field, were performed last year at Harvard, the Massachusetts Institute of Technology, and Ludwig Maximilian University of Munich. Khatami was also a part of the MIT study, which was published in Science last year.