Center for Quantum Phenomena Seminars

The Spin Hall Effect (SHE) offers promising applications in spintronic devices, such as controlling magnetization in magnetic random-access memory (MRAM) [1,2], spin Hall nano-oscillators (SHNOs) [3], spintronic terahertz emitters (STEs) [4], and magnetic sensors [5]. To further enhance the efficiency and material flexibility of such devices, orbitronics has emerged as a compelling direction by utilizing the orbital angular momentum (OAM) of electrons, which can generate sizable orbital currents even in materials with weak intrinsic spin–orbit coupling (SOC). Motivated by this, we adopted an approach based on the orbital Hall effect (OHE) [6–8], which is analogous to the SHE but produces an orbital current from a charge current without requiring materials with strong SOC. Theoretical predictions suggest that the orbital Hall conductivity (OHC) can be an order of magnitude higher than the spin Hall conductivity (SHC), enabling the development of more efficient devices [9].
In my talk, I will focus on three key questions:

1. How can we experimentally distinguish the SHE from the OHE, even though both effects exhibit the same symmetry?
2. Does orbital transport behave in the same way as spin transport?
3. Does the large OHC, compared to the SHC, truly improve the performance of spin–orbit torque (SOT) MRAM and SHNO devices?

To address these questions, we investigate the OHE in low-SOC Nb and Ru systems, demonstrating significant damping-like orbital Hall torque efficiencies in Nb/Ni and Ru/Ni bilayers. Notably, Nb/Ni exhibits a sign reversal in orbital torque compared to Nb/FeCoB, highlighting distinctive OHE characteristics [8]. Furthermore, we demonstrate that Ru/Pt OHE layers achieve a 30% enhancement in orbital Hall torque and a 20% reduction in switching current density compared to conventional SHE-based Pt layers in SOT-MRAM devices [10].
Using a similar approach, we achieved a 50% reduction in the auto-oscillation threshold current in our orbitalassisted SHNOs compared to purely SHE-dominated nano-oscillators. This observation is in excellent agreement with theoretical predictions regarding orbital-to-spin conversion mechanisms, as well as with the calculated signs of orbital and spin Hall conductivities in our oscillators [11]. Finally, I will discuss the ballistic transport of orbital angular momentum, where we observed a 3 fs/nm delay in the terahertz emission spectra with increasing Ru thickness in Ni/Ru terahertz emitters [12].
These findings provide compelling experimental evidence for the active role of orbital currents in driving magnetization dynamics and reinforce the potential of orbitronics to complement—or even replace— conventional spintronic approaches in future device architectures.

References:
[1] J. Sinova et al.; Rev. Mod. Phys., 2015, 87, 1213.
[2] S. Husain, R. Gupta, et al.; Appl. Phys. Rev., 2020, 7, 041312.
[3] S. Jiang et al.; Appl. Phys. Rev., 2024, 11, 041309.
[4] R. Gupta et al.; Adv. Optical Mater., 2021, 9, 2001987.
[5] S. Koraltan, R. Gupta, et al.; Phys. Rev. App., 2023, 20, 044079.
[6] D. Go et al.; Phys. Rev. Lett., 2018, 121, 086602.
[7] S. Ding et al.; Phys. Rev. Lett., 2020, 125, 177201.
[8] A. Bose, R. Gupta, et al.; Phys. Rev. B, 2023, 107, 134423.
[9] L. Salemi et al.; Phys. Rev. Mat., 2022, 6, 095001.
[10] R. Gupta et al.; Nat. Commun., 2025, 16, 130.
[11] R. Gupta et al.; Under preparation, 2025.
[12] R. Gupta et al.; Under preparation, 2026.

The superconducting phase mode is a collective mode that is gapped in three-dimensional superconductors, but gapless in two-dimensional superconductors. This mode is not infrared active in the far-field, but can be probed with near-field optical measurements. We develop theory showing that the near-field nature of on-chip time domain THz spectroscopy can access this mode via superconducting microcavity resonances - which we call `Cooper plasmons'. We experimentally show spectroscopic signatures of two Cooper plasmons in a superconducting NbN film and use these Cooper plasmons to report the real and imaginary parts of the self-energy across a thermal metal-to-superconductor transition. We discuss applications of Cooper plasmons in other superconducting systems, as well as ways to avoid parasitic Cooper plasmons in superconducting circuit engineering.

As transistors approach atomic limits, heat dissipation has become the defining constraint of modern computing. My research asks a different question: can heat itself be used to compute? Using correlated quantum materials such as vanadium dioxide (VO2), we explore how electronic phase transitions driven by the flow of heat and charge, generate nonlinear dynamics that resemble neural behavior. I will discuss our recent experiments revealing spiking, synchronization, and stochasticity in Mott oscillators, as well as collective switching in thermally coupled device networks. These studies uncover how local phase transition fluctuations and mesoscale heat transport give rise to emergent order and functional computation. By linking atomic-scale phase transitions to network-level information processing, this work outlines a physical pathway from atoms to bits, pointing toward a thermodynamic framework for intelligent, energy-aware electronics.

Teaser line:
From atomic phase transitions to emergent intelligence, see how computation can arise from the physics of heat itself.

Speaker Bio:
Erbin (Ben) Qiu is a postdoctoral researcher in the Department of Physics at the University of California, San Diego. He received his Ph.D. in Electrical and Computer Engineering from UC San Diego in 2024, where his dissertation focused on collective dynamics in coupled spiking oscillators based on quantum material. His research combines condensed-matter physics, materials science, device engineering, and neuroscience to develop energy-efficient computing paradigms that harness thermodynamic processes for intelligent function. His work has appeared in Advanced Materials, PNAS, Applied Physics Letters, and Nature Communications.