세미나 일정:

Iyyappa Rajan Paneer

Affiliation: YST Research Fellow, Asia Pacific Center for Theoretical Physics (APCTP), POSTECH Campus, Korea.

Date: March 16, 2021; 15:00 – 16:00

High-Temperature Thermoelectric Materials for Future Energy Conversion Applications – A Combined First-principles and Boltzmann Transport Formalism approach

While most of the thermoelectric materials work well only at low and mid temperatures, high-temperature thermoelectric materials are equally important for future energy conversion applications. The applications include but not limited to the operation of deep spacecraft missions and the conversion of the waste heat from the nuclear reactors and high-temperature industrial reactors into electrical power generation. To accomplish this demand, I will talk about a few insights of designing high-temperature thermoelectric materials for energy conversion from my perspective. This presentation is based on the results obtained from the first-principles density functional theory (DFT) calculations and Boltzmann transport formalism along with an experimental literature validation wherever possible. In this talk, I will also elaborately discuss the transport properties such as Seebeck coefficient, electrical and thermal conductivities which are calculated using the various physical parameters of materials to obtain the optimum thermoelectric power-factor and figure-of-merit.

Lieven Vandersypen

Affiliation: Delft University of Technology

Date: March 23, 2021; 17:00 – 18:00

Quantum Computation and Simulation – Spins inside

Excellent control of over 50 quantum bits has been achieved, but can we scale up quantum computers to solve relevant problems? Quantum bits encoded in the spin state of individual electrons in silicon quantum dot arrays have emerged as a highly promising avenue. In this talk, I will present our vision of a large-scale spin-based quantum processor, and our ongoing work to realize this vision. I will also show how the same quantum dot arrays offer a powerful platform for analog quantum simulation of Fermi-Hubbard physics and quantum magnetism.

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Jose Reina Galvez

Affiliation: CFM – Materials Physics Center

Date: February 17, 2021; 17:00 – 18:00

Floquet theory applied to ESR

Electron spin resonance (ESR) with the STM is a revolutionary technique that is permitting us to have unprecedented insight into quantum dynamical processes. To really achieve this goal, theory can be instrumental. To this end, we have developed a theory that computes the time-dependent electron current through a quantum system. The theory is based on the Floquet formalism that allows us to treat a complex time dependence, and it also uses non-equilibrium Green’s functions to deal with realistic bias drops in the quantum system. We can then easily identify the behavior of the electronic current with the actual quantum dynamics.

In this talk, I will first present the results of a spin-1/2 driven by a harmonic time-dependent bias. We will analyze the results of these calculations, showing the main ingredients of the theory. Then, we will move to two spins interacting via an exchange interaction and reproduce the experiments by Bae et al 2018. We evaluate the von Neumann entropy and show that entanglement is needed, but it is rather constant for all the cases evaluated here. Moreover, we show why the Rabi’s frequencies of the different transitions are changing, which gives us a handle on the actual parameters controlling the dynamics of this two-spin system. We will briefly show other results on spin-1 and spin-2 systems to show the versatility of our approach as well as its predictive power.

Sander Otte

Affiliation: Delft University of Technology, Netherlands

Date: February 26, 2021; 16:00 – 17:00

Free coherent evolution of a coupled atomic spin system initialized by electron scattering

Observing the free evolution of a coupled spin system is an essential step towards studying collective quantum spin dynamics, as well as gaining insight into the fundamental mechanisms leading to spin excitation. Here, we combine pump-probe and ESR techniques with STM to study the free evolution of a single atomic spin depending on its level of entanglement with a second one. We build TiH dimers on MgO/Ag(100) in which the two spins are inherently detuned. We then make use of the magnetic interaction with the STM tip to tune the level of entanglement between the two spins: usingESR, we characterize the energy diagram of the dimer and identify the tip height at which both spin precess at the same frequency. Subsequently, we use a pump-probe scheme to, first, initialize the system via an electron induced spin excitation and, second, study the free evolution of the spin under the tip. We show that only when the two spins entangle, the excitation is swapped back and forth at a frequency that is given by their coupling strength. These results provide insight into the locality of electron-spin scattering: only the spin directly underneath the tip is affected, irrespective of its global quantum state.